Accident Analysis and Prevention 44 (2012) 19–29
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Accident Analysis and Prevention
journal homepage: www.elsevier.com/locate/aap
How to make more cycling good for road safety?
Fred Wegman a,∗ , Fan Zhang a , Atze Dijkstra b
a
b
SWOV Institute for Road Safety Research, Delft University of Technology, The Netherlands
SWOV Institute for Road Safety Research, The Netherlands
a r t i c l e
i n f o
Article history:
Received 31 October 2010
Accepted 8 November 2010
Keywords:
Cyclist safety
Bicycle crashes
Prevention
State-of-the art review
a b s t r a c t
This paper discusses the current level of the road safety problems of cycling and cyclists, why cyclists run
relatively high risks, and why cyclists may be considered as ‘vulnerable road users’. This paper is based on
peer-reviewed research which give some idea how to reduce the number of cyclist casualties. However,
this research is rather limited and the results cannot (easily) be transferred from one setting or country
to another: generalization of results should only be done with the utmost care, if it is to be done at all.
Interventions to reduce cyclist casualties worldwide seem to be of an incidental nature; that is to say,
they are implemented in a rather isolated way. In a Safe System approach, such as the Dutch Sustainable
Safety vision, the inherent risks of traffic are dealt with in a systematic, proactive way. We illustrate how
this approach is especially effective for vulnerable road users, such as cyclists. Finally, the paper addresses
the question of whether it is possible to make more cycling good for road safety. We conclude that when
the number of cyclists increases, the number of fatalities may increase, but will not necessarily do so,
and the outcome is dependent on specific conditions. There is strong evidence that well-designed bicycle
facilities—physically separated networks—reduce risks for cyclists, and therefore have an impact on the
net safety result, for example if car-kilometres are substituted by bicycle kilometres. Policies to support
cycling should incorporate these findings in order to make more cycling good for road safety.
© 2010 Elsevier Ltd. All rights reserved.
1. Introduction
If we compare bicycle use among different countries there are
enormous differences: from near absence to widespread use in
countries such as the Netherlands. The amount of cycling is often
partly determined by a country’s geography (hills and mountains)
and its climate (temperatures, snowfall). There are countries where
cycling is practiced for recreation. And, finally, there are countries in
which cycling is a substantial part of everyday life. Although cycling
activities also take place in rural areas, the majority of the bicycle
kilometres in such countries are travelled in towns and cities, and
over relatively short distances.
Differences in bicycle use can be observed in bicycle culture,
purpose of bicycle use, the position of the cyclist in traffic, and the
measures that have been taken to make cycling safer.
Many different arguments can be used to promote cycling. An
important distinction that must be made is whether cycling is
recreational, or whether it is a means of transport to travel from
A to B. Some of the arguments used are: cycling is healthy, cycling
is good for the environment if it takes the place of motorized journeys, cycling makes a contribution to the prevention of congestion
∗ Corresponding author.
E-mail address: [email protected] (F. Wegman).
0001-4575/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.aap.2010.11.010
because cyclists take up less space than (parked) cars, and cycling is
cheaper than travel by passenger car or public transport. Compared
to walking, cycling increases the distances that can be covered and
in developing countries it can contribute to the economic development and aid the fight against poverty. Nowadays, more and more
governments, cities and villages, communities encourage their citizens to cycle.
One important objection can be made against promoting
cycling: it is rather dangerous. As a direct consequence of the laws
of (bio)mechanics and the fragility of the human body cyclists
are vulnerable in traffic. Cyclists fall easily and can sustain serious injury. In crashes, other than sometimes by a bicycle helmet, a
cyclist is unprotected. Brain damage is a serious and frequent injury,
often sustained by young people in particular. When a cyclist is
injured in a crash with a motorized vehicle travelling at high speed,
kinetic energy is processed. Furthermore, a cyclist can lose control of the bicycle, take a fall, and be injured, especially if a cyclist
is inexperienced or when obstacles play a role. Often cyclists fail
to obey the traffic rules and show unexpected behaviour in the
eyes of other road users. The consequence is that cyclists have a
relatively high crash rate compared to that of pedestrians and particularly that of drivers. Because cycling is relatively risky we have
to ask ourselves whether or not it will increase injuries and fatalities if a government is successful and more people do indeed use
a bicycle.
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F. Wegman et al. / Accident Analysis and Prevention 44 (2012) 19–29
Kilometers of cycling
3
2,5
2,5
2
1,6
1,5
0,9
1
0,5
0,7
0,4
0,1
0,1
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0,2
0,4
0,5
0,9
0,7
0,5
0,2
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Fig. 1. Kilometres cycled per inhabitant per day in some European countries and in the USA (from Pucher and Buehler, 2008; 2000-data from the European Commission).
1.1. Cycling worldwide
The use of a bicycle as a transport mode differs tremendously
among countries: from practically non-existing up to almost 30%
of all trips being made by bicycle in, for example, the Netherlands.
The number of bicycle kilometres travelled per inhabitant varies as
can be seen in Fig. 1.
Bicycle use is increasing (slightly) in some European countries,
more specifically in cities. Many countries experienced a steep
decrease in bicycling after the Second World War in conjunction
with an increase in motorization (FHWA, 2010). In other countries,
such as China, the cycling tradition has been taken over by motorized vehicles. Bicycle use is also determined by the geography of a
country and its climate. A bicycle can be used for recreation or be
substantial part of the modal split. Sometimes cycling is restricted
to certain age groups (young and old), to lower socio-economic
classes, and in many countries worldwide cycling is a male activity. There is such a variety in bicycle traffic that to sketch a mean
picture would obscure reality. A fact or finding from one country
can only be transferred to other countries with great caution, if this
should be done at all.
The most common problem for cyclists worldwide is that our
modern traffic system is designed largely from a car-user perspective, which results in a lack of coherent planning of route networks
for cyclists (ETSC, 1999). Nor does the system take the main characteristics of cyclists into account: a cyclist is vulnerable (in a crash),
flexible (in behaviour), instable (may fall off the bike), inconspicuous (difficult to see), has differing abilities (due to a wide range of
the population), is conscious of effort (i.e., highly motivated to minimize energy expenditure), and sometimes seen as intruders in the
traffic system, rather than as an integral part. These key problems
also occur in combination.
1.2. Risk
Different countries worldwide indicate that cyclists have a relatively high crash rate compared to car drivers and pedestrians.
As will be discussed in Section 2.1, we know that bicycle crashes
suffer from underreporting, especially when non-fatal injury is concerned.
In general, we find higher crash rates for cyclists than for drivers.
A difference of a factor of more than 10 was found in a relatively
old study in Europe (PROMISING, 2001). More recent developments
in the Netherlands indicate that in a 20 year period (1988–2009)
the number of cyclist fatalities was halved and that of car drivers
and passengers reduced by 55%. At the same time the number of
kilometres travelled by cyclists increased by about 30% and that of
cars, an important crash opponent, increased by almost 80%. This
means that risks of cyclists (per kilometres travelled) decreased less
than risks of car occupants during this period.
It is important to understand that when comparing transport
modes risk factors obscure age effects. Because especially the young
and the elderly have relatively high crash rates (see also Table 2),
the mean crash rate for a country is strongly influenced by the share
of these high risk groups in the total number of fatalities.
1.3. Combination of numbers and risks
Fig. 2 shows the relation between fatality rates and bicycle usage
for a number of European countries (as referred to by van Hout,
2007). The graph depicts the relationship between the distances
travelled by bicycle (kilometres travelled per person per year) in
several European countries with the fatality rate (number of fatalities per kilometre travelled) in those countries. A regression line
can be drawn which suggests that countries with a lot of bicycle
traffic have a relatively low fatality rate (e.g. the Netherlands and
Denmark) while countries where inhabitants do not cycle much
(a dozen of kilometres per inhabitant per year only) face relatively high fatality rates. However, if we compare Portugal, Spain,
France and the UK, all counties with less than 100 km travelled
per year, we observe a wide variance. We, therefore, apparently
need more factors than just the number of kilometres travelled to
explain differences in fatality rate. The graph in Fig. 2, however, is
very important indeed in scientific literature (Jacobsen, 2003). This
intriguing relationship will be discussed in Section 5.
Three different groups of countries can be distinguished according to different levels of bicycle use and fatality rate:
– Denmark and the Netherlands, in which the distance travelled by
bicycle is relatively high and the fatality rate is relatively low.
– Southern European countries like Portugal and Spain, in which
the distance travelled by bicycle is relatively low, but the fatality
rate is relatively high.
– Countries (almost all other) in which the distance travelled by
bicycle and number of trips are low, but the fatality rate for
cyclists is also low.
However, in the latter group, we can observe major differences;
for example, between the UK and Sweden on the one hand, and
Austria on the other hand. This illustrates that the distance travelled cannot explain all differences in fatality rates; an interesting
observation for further research.
Fatalities per billion kms travelled (cyclists)
F. Wegman et al. / Accident Analysis and Prevention 44 (2012) 19–29
21
250,00
200,00
Portugal
150,00
Spain
100,00
Austria
Finland
50,00
Italy
France
UK
0,00
0
Belgium
Ireland
100
200
Netherlands Denmark
Germany
Sweden
300
400
500
600
700
800
900
Distance travelled by bicycle per person per year (kms)
Fig. 2. Relation between fatality rates and bicycle usage for European countries based on IRTAD-data (referred to by van Hout, 2007).
1.4. Cycling benefits: improved health, and reduction of air
pollution, congestion, and noise
Cycling advocates never tire of stating the great advantages of
cycling, especially compared with travelling by car. For example,
they refer to the health benefits if cycling results in incorporating exercise into someone’s life. Using active transportation modes
results in lower obesity prevalence for populations and better cardiovascular health. In their review Pucher et al. (2010) suggest that
‘the combined evidence presented in these studies indicates that
the health benefits of bicycling far exceed the health risks from
traffic injuries, contradicting the widespread misperception that
bicycling is a dangerous activity’. However, In his book ‘Pedaling
revolution’ (2009) Mapes states that Pucher ‘.. makes no pretense
of being the detached academic’. The sources supporting the claim
that cycling is healthy need some scrutiny before it can be accepted.
This can be illustrated with a frequently cited conclusion of Bassett
et al. (2008) that there is an inverse association between active
transportation (walking, biking and public transport/transit use)
and obesity rates in the countries studied: the more walking,
cycling and public transport use, the lower the obesity rates. However, these authors, and rightly so, do not claim to have established a
causal relationship, because they were not able to control for other
factors that could influence obesity rates such as other physical
activity domains or international differences in ‘energy intake’.
A Dutch study (de Hartog et al., 2010) published findings
using results of international literature which were applied to the
Netherlands. The study assumes that 500,000 people (ages 18–64)
make a transition from driving to bicycling for short trips on a daily
basis, and concludes that the beneficial effects of the increased
physical activity amount to 3–14 life months gained, which is substantially higher than the potential mortality effect of the increase
in inhaled polluted air (0.8–40 days lost) and the increase in traffic crashes (5–9 days lost). Therefore, it concludes that the health
benefits of cycling outweigh the health risks. This study, however,
excluded high risk groups (cyclists younger than 18 and older than
64) and including theses groups will definitely influence the end
result (see also Section 5).
The advantages of cycling are often cited in policy documents
promoting cycling all over the world (Dekoster and Schollaert,
1999; CEMT, 2004; Racioppi et al., 2005; Austroads, 2005; USDOT,
2010; WHO, 1999). The supposed advantages are not only related to
individual health improvements, but also to less air pollution, less
congestion and less noise. Although, these claims are supported
by logical assumptions (e.g. bicycle traffic will replace motor-
ized traffic), the reported results are dependent on the conditions
under which the change takes place. This limits generalization, and
therefore reported results are not universally valid. Designers of
strategies to promote cycling should always keep that in mind.
2. Road safety problem of cycling
The considerable differences among countries do not allow for a
meaningful comparison of the well-known crash characteristics as
reported in police or hospital reports. Three issues are problematic
in drawing conclusions from police reports: underreporting, age
distribution, and some fundamentals in the causes of crashes with
cyclists in relation to the key problems of this transport mode.
2.1. Underreporting of crashes
Reliable, accurate data can help to identify road safety problems, risk factors and priority areas. Good data are required to
adequately document the magnitude and nature of the road safety
problems. A survey by the World Health Organization (WHO, 2009)
showed that approximately half of the 178 participating countries only use police records to collect data on fatalities, the other
half use different sources, and only 2% of the countries have no
data at all. International comparison of fatality data requires a
standardized definition of a road traffic fatality. However, countries use a wide range of definitions. Data on severely injured lack
even more in quality, reliability, and international agreement on
definitions (Derriks and Mak, 2007). Underreporting must be considered a crucial problem in relation to road safety data, more
specifically with crash and injury data. Underreporting not only
complicates international comparisons, but it also biases problem
analysis within countries. The WHO (2009) concludes that among
the factors that can affect the quality of reported data are political influences, competing priorities, and availability of resources.
Derriks and Mak (2007) are more specific and identify low perceived relevance of this task by the police and the weaknesses in
the error-prone processes (organisational, administrative, ICT) in
building and maintaining a good database of road crashes. A few
countries have made initial attempts to link data sources, such
as police and hospital data, in order to overcome at least part of
this underreporting problem. Problems in data systems, especially
underreporting, affect cyclists more than any other transport mode.
In other words: underreporting is selective and biased.
For most conflict types, the number of casualties registered by
the police are reasonably accurate. Although underreporting is a
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F. Wegman et al. / Accident Analysis and Prevention 44 (2012) 19–29
Table 1
Percentages of casualties reported in three different severity classes (fatality, severe injury and slight injury) in Great Britain and the Netherlands.
Great Britain
Passenger car
Motorcycle/scooter
Bicycle
Pedestrian
Total
The Netherlands
Fatal (%)
Severe (%)
Slight (%)
Fatal (%)
Hospitalized (%)
Slight (%)
100
100
100
100
100
89
70
33
85
76
77
51
21
67
62
96
94
86
90
93
92
63
31
56
60
33
13
4
20
13
Source: ERSO, 2008a; SWOV-AVV.
well-known phenomenon, it has not been very well researched.
We find incidental evidence that more severe crashes and crashes
involving motorized vehicles are better reported than less severe
crashes and crashes involving ‘vulnerable road users’. Crashes
involving cyclists tend to have a relatively low reporting rate compared with other transport modes. These crashes, even those with
serious injury, are often not reported to the police. Therefore, the
real numbers of injured are grossly underestimated by the police.
Comparing serious traffic injuries (inpatients with a maximum
abbreviated injury score with a minimum of 2) in the hospital data
base with police data in the Netherlands (Reurings and Bos, 2009)
led to the conclusion that the police records only contained 59%
of the seriously injured casualties in motor vehicle crashes and
only about 4% of the casualties in which no motor vehicle had been
involved (e.g. crashes between two bicycles).
Data from Great Britain and the Netherlands clearly shows
that the underreporting rate increases as the victims’ transport
mode changes from passenger car to cyclist and as injury severity
increases (Table 1). For all three severity classes, casualties among
cyclists are reported far less frequently than casualties among other
road users. Bicycle crashes in which no other vehicle was involved
are heavily underreported. Examples of such crashes are accidents
in which the cyclist fell, slipped, or collided with an obstacle.
Underreporting is a serious problem and has not yet been
thoroughly researched. The international safety data community
(IRTAD, WHO, European Union) has recommended that this phenomenon be studied further and that proper approaches to reduce
underreporting be developed. In the meantime, cyclists injury data
should be used with extreme caution.
2.2. Age distribution of cyclist casualties
According to ERSO (2008b) in 2005, 44% of all bicycle fatalities
in Europe were cyclists older than 60 (see Fig. 3). In Finland and
Sweden more than 60% of bicycle fatalities were older than 60.
Table 2
Casualties in the Netherlands per age group per billion kilometres travelled for
bicycles and cars in 2008.
Age category
Bicycle
Car
Ratio (B/C)
0–11
12–19
20–29
30–39
40–49
50–59
60–74
75+
All ages
3.6
6.6
4.5
3.1
5.9
8.3
19.4
102.4
10.6
0.5
4.9
5.5
2.1
0.9
1.3
1.9
12.3
2.3
7.9
1.4
0.8
1.5
6.4
6.6
10.4
8.4
4.7
Source: Ministry of Transport—Statistics Netherlands.
The highest proportion of cyclist fatalities among children is found
in the ages between 6 and 14. About 14% of the fatalities in this
age group were cyclist fatalities; twice the average percentage for
all age groups combined. Children between 10 and 14 years old
also have the highest proportion of cyclist casualties: 30% of the
casualties in this age group were cyclist casualties.
However, comparison of these figures with those from other
parts of the world, might lead to an entirely different conclusion.
Table 2 shows the estimated number of casualties in the
Netherlands in 2008 per age category per billion kilometres travelled. As we can see from the table, for all ages there are about 4.7
times more casualties per kilometre travelled for bicycles than for
cars. Except for young adults (age 20–29), cycling is riskier than
driving a car for all age groups, with about 10 times more casualties among the 60–74 years old and 8 times more casualties among
the 0–11 years old and among those older than 75.
The differences in fatality rates between bicycle and car on short
trips are probably overestimated, because long car trips on motorways are usually relatively safe and therefore they reduce the mean
crash and fatality rates for cars. In 1999, a European Commission
50
45
40
35
30
25
20
15
10
5
0
0-14
15-24
25-39
40-59
Fig. 3. Bicycle fatalities for different age groups (total of all columns = 100%).
Source: ERSO, 2008b.
60+
F. Wegman et al. / Accident Analysis and Prevention 44 (2012) 19–29
23
study compared the risk of a fatal crash for car drivers and cyclists in
the Netherlands. In this report Dekoster and Schollaert (1999) used
two correction factors, which meant that motorway journeys were
excluded and the hazard which motorists represent for pedestrians and cyclists was included. Their calculation showed a similar
risk of accident per million kilometres for cyclists (21.0), and for car
drivers (20.8).
2.3. Causes of risks and injuries: vulnerability, incompatibility
and behaviour
A cyclist-only crash only provides a small amount of kinetic
energy due to the relatively small masses and low speeds, whereas
the kinetic energy of motor vehicles is much greater. The different
amounts of kinetic energy when different types of road users use
the same traffic area and collide, combined with the differences
in protection, result in incompatibility. Elvik (2010) calculated the
fatality rates that can be attributed to incompatibility between different types of vehicles and groups of road users in Norway. Elvik
focused on the ratio of the number of injured road users using other
modes of transport in relation to injured road users in their own
mode of transport. This ratio reflects the risks for the other types
of road users. For the period 1998–2005 that ratio was 0.05 for
cyclists, which mean that for each cyclist who was injured in a
crash, there were 0.05 people belonging to other groups of road
users who were injured in that same crash. In comparison with the
cyclists, the ratio for other vehicles is higher (varying from 0.27
for car occupants, to 1.45 for van and bus occupants, to 3.46 for
truck occupants). In contrast, the ratio for pedestrians was much
lower: 0.03. Wegman and Aarts (2006) made a similar calculation
for fatalities and severely injured and found that the incompatibility factor (casualties in the weakest party divided by casualties
in the strongest party) is much higher (bicycle–car ratio = 150:1).
This means that for one car casualty there are 150 bicycle casualties in car-bicycle crashes. Therefore, vulnerable road users such as
cyclists run an even higher risk of getting injured in a crash with a
‘disproportionally strong crash opponent’.
A second major safety problem for cyclists seems to be the
single-vehicle crash. It is a misunderstanding that the safety problem of cyclists is just a problem of crashes between motorized
vehicles and bicycles. Falling off a bicycle, for whatever reason,
hitting an obstacle or leaving the road can also result in (serious)
injury. Dutch statistics illustrate this (Schepers, 2008).
Very little research is available about the effects of cyclist
behaviour or cyclist characteristics on crashes and the risk of cyclist
crashes. We have, for example, strong evidence for car drivers on
the relationship between age/experience and their fatality rate, we
know quite a lot about the relationship between fatality rate and
blood alcohol content (BAC), we have some crash prediction models
for different road types and design characteristics, but for cyclists
this type of information has not been very well researched, if it
exists at all. We recommend collecting this information for consideration when policies are designed to promote cycling and improve
cyclist safety.
3. Interventions to reduce risks and severity of bicycle
crashes
Good evidence that cycling is relatively hazardous and a growing interest in many parts of the world in promoting cycling raises
the question of how to reduce the hazards of cycling and how
to minimize the negative consequences of a crash. Our survey of
peer-reviewed research indicated that a rather limited amount of
research on cyclist safety is available. Before applying solutions, it
is important, once again, to make clear that results cannot (easily)
Fig. 4. Different packages of measures for different densities of facilities.
Source: Adonis (Road Directorate Denmark, 1998).
be transferred from one setting or country to another. Generalization of results should be done with utmost care, if it is to be done
at all. This seems to be a major hurdle in making progress towards
a better understanding of bicycle safety.
Some documentation of interventions to improve safety is available for cyclists from North-Western Europe (e.g. Pucher and
Buehler, 2008; Pucher et al., 2010; FHWA, 2010), mainly from
countries such as the Netherlands, Denmark and Germany. Only
a limited number of studies into the safety effects of interventions have been carried out and very few of them meet rigorous
standards. Two subjects related to cyclist safety that have been discussed thoroughly in peer-reviewed literature are bicycle helmets
and roundabouts.
If countries were to be categorized into different groups according to their current bicycle usage and existing level of bicycle
facilities, it has been hypothesized that these differences would
result in different investment schemes and types (Road Directorate
Denmark, 1998). If neither policy nor facilities do yet exist, then a
start will be made by taking elementary simple isolated low-cost
measures. The more cycling and the higher the density of facilities,
the more advanced measures will be contemplated and implemented (Fig. 4). These more advanced packages of measures may
be part of a systemic and network-wide approach. One can assume
that the density of facilities is proportional to the share of cycling
in the modal split. This assumption was tested in a European ‘of
the art’ project on cyclists and pedestrians, called Adonis. Adonis
not only included so-called best practices for cyclists, but also for
pedestrians. In Fig. 4, the share in modal split is on the horizontal
axis and the density of facilities on the vertical axis. Three areas
have been distinguished in this figure:
• area 1 with low density of facilities and a small share of cycling
or walking;
• area 2 with medium density of facilities and medium share of
cycling or walking;
• area 3 with high density of facilities and large share of cycling or
walking.
For each area Adonis selected a package of measures for cyclists
and concluded that investments could be found at the diagonal of
this matrix rather than elsewhere.
This is an interesting hypothesis, which certainly needs further testing: certain intervention types seem to correlate with
the development stage of cycling. If this hypothesis is confirmed,
two interesting conclusions can be derived. First, if we use results
from evaluation research in package 3, we cannot simply transfer
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F. Wegman et al. / Accident Analysis and Prevention 44 (2012) 19–29
these results to settings characteristic of packages 1 and 2. Second, if investments in the countries which were included in this
study—Belgium, Denmark, the Netherlands and Spain—could be
placed in these three categories, could this be the result of a logical
step-by-step approach which should be followed by other jurisdictions elsewhere? If the answer is positive, then we have a logical
step-by-step approach that can generally be followed.
3.1. Interventions to reduce crash rate and injury rate for cyclists
Traffic safety has three primary dimensions: exposure (to risk),
risk (of having a crash given a certain amount of exposure), and
consequences (e.g. injury given a crash). When it comes to cycling
and cycling policy, many countries and cities seem to aim for an
increase in the number of kilometres travelled by bicycle rather
than a reduction. Increasing the bicycle kilometres travelled can
only reduce exposure to risk if potential conflicts between cyclists
and other road users are prevented. This can be done by separating
different modes of transport in space or in time. If this cannot be
done, motorized vehicles need to reduce their speed to a ‘impact
speed’ when meeting with a cyclist. A safe impact speed is such
a speed that the risk of serious injury is very limited. Although
we have some indication about safe impact speeds for pedestrians
(Tingvall and Haworth, 1999), we do not have specific information
on impacts between cyclists and motorized vehicles. The different impact manoeuvres do not allow the simple assumption that
the same impact speed is safe for both pedestrians and cyclists
(Rodarius et al., 2008). Therefore, further research into safe impact
speeds between cars and cyclists is recommended.
Besides preventing crashes between cyclists and other road
users, we must address the safety problem of single-vehicle crashes
for cyclists. This crash type is characterized as follows: the more
kilometres travelled, the higher the exposure to risk. Relatively
little scientific literature exists about this crash type (Schepers,
2008). We can assume that the risk of such a crash increases with
poor vehicle handling and hazardous conditions (bad road surface,
obstacles, etc.). This crash type seems to be underestimated and
better analysis of these crashes is advisable before coming up with
solutions.
3.2. Reducing the injury rate: bicycle helmets and vehicle design
of cars and trucks
The only protective device available for cyclists is the bicycle
helmet. It is widely accepted that a properly designed helmet provides very good protection for the most vulnerable part of the body,
the head, against being severely injured in a crash (SWOV, 2009).
SWOV has calculated the maximum effect of a bicycle helmet to be
approximately a 45% reduction of the risk of head and brain injury
when a good helmet is worn correctly. However, if one needs to
use ‘controlled trials’ as the only way of finding proper scientific
evidence, some doubts remain. Whereas the helmet generally is
compulsory for participants in sporting events, wearing a helmet
for bicycle touring or bicycle rides in general is still optional in most
countries. This introduces another heavily debated issue: how conclusive are studies that evaluate the safety effects of introducing
helmet legislation?
Some cyclists are opposed to the helmet as it conflicts with their
feeling of freedom that goes with riding a bicycle, or because it is
unsightly, uncomfortable, or unnecessary over short distances. Others are strongly in favour of it as it provides good head protection.
One of the often-heard arguments is the negative impact of compulsory helmet use on the use of bicycles, and this is supported by
an unmistakable decline in the use of bicycles after the introduction of compulsory helmet use in provinces in Australia and Canada.
Presently, the final conclusion must be that deciding on helmet leg-
islation is a political decision, given that such a decision will have
positive and negative effects on cycling and cyclist safety.
Injuries to cyclists (and pedestrians) can be reduced by better design of cars and heavy vehicles. Design measures include
crash-friendly car fronts, and side-underrun protection on lorries
(Wittink, 2001). The design mainly focuses on the points of the body
where cyclists or pedestrians are hit by cars. Lorries can be made
much safer for third parties by the application of adequate protection around the vehicle. Such protection prevents the dangerous
underrun of, for instance, cyclists and riders of other two-wheeled
vehicles.
In some of the crashes between heavy goods vehicles and
two-wheelers, injury severity can be reduced by side-underrun
protection. The number of traffic fatalities in urban areas due to
crashes of this type could be reduced. For cyclists, but also for
moped riders and pedestrians, closed side-underrun protection
(with full screens) on lorries is more effective than open protection
(with two or three bars only). Both open and closed side-underrun
protection appeared in the top 10 list of relevant and cost-effective
measures in the Netherlands to reduce the number of casualties
as a result of crashes involving lorries (van Kampen and Schoon,
1999).
A study of collisions between cars and vulnerable road users
such as pedestrians and cyclists (Rodarius et al., 2008) showed that
not all safety measures for pedestrians are effective for cyclists.
This can, for example, be observed in collisions of pedestrians and
cyclists with passenger cars. The study indicated that while head
injury is the most dominant (life-threatening) injury, the impact
point of a cyclist’s head is higher than that of a pedestrian’s head.
Equipping cars with exterior airbags should therefore be considered, and cost-benefit analyses (CBA) should be made to support
such decisions.
3.3. Reducing the crash rate: infrastructure measures
Crashes between motorized vehicles and cyclists are a traditional and well-known crash type. Two methods can be used to
reduce the crash rate for cyclists: for road stretches methods can be
developed to physically separate both modes of transport (bicycle
tracks, etc.). At intersections, speed reduction of motorized vehicles must be aimed for. Research is available into two types of
countermeasures: bicycle tracks and roundabouts.
When separating cyclists form motorized traffic it is almost
inevitable to use an area-wide approach and to relate the planning
of infrastructure to the nature of bicycle trips. This introduces the
level of planning and design of comprehensive packages of measure to improve cyclists’ safety. But, once more, very little research
is available on this subject.
Bicycle facilities (bicycle lanes and bicycle paths) on road segments of roads where cars can drive relatively fast (>50 km/h)
reduce risks for cyclists (SWOV, 2008). Also cyclists benefits from
traffic calming measures, reducing speeds of motorized traffic to
less than 30 km/h.
Converting three-leg or four-leg intersections into roundabouts
is good for road safety. Although to a lesser extent than for motorized vehicles, this is also the case for cyclists. A roundabout reduces
the number of potential conflict points and, if well-designed, it
reduces the severity of a conflict because of reduced approach and
travel speeds. Studies report substantial reductions in the number
of crashes and risks as a result of the construction of roundabouts
(Elvik et al., 2009). As is usually the case, reduction of more severe
injuries is estimated to be higher than of less severe injuries: a
two-thirds reduction of fatal crashes and a 46% reduction of injury
crashes; the reductions in urban areas are lower (25%) than in rural
areas (69%). There is some indication (from the Netherlands and
Denmark) that these very positive results are not entirely true for
F. Wegman et al. / Accident Analysis and Prevention 44 (2012) 19–29
cyclists. A Belgian study even found that roundabouts increased
bicycle injury crashes (Daniels et al., 2008). It is recommended to
do further research into bicycle safety on the relationship between
the design features of roundabouts and bicycle facilities and actual
behaviour of motorists and cyclists. We may observe complicated
interactions between cyclists and motorized traffic; more specifically related to how cyclists on a roundabout intersect with traffic
entering or leaving a roundabout. The design of the roundabout and
its bicycle facilities in relation with the priority regulations seem
to be the relevant variables (Hels and Orozova-Bekkevold, 2007;
Sakshaug et al., 2010).
3.4. Improving poor cyclist behaviour by education and
enforcement
In order to improve their understanding of the traffic rules
and regulations and to improve their cycling skills, Dutch, Danish, English and German children receive some training in safe
and effective cycling techniques as part of their regular school curriculum (FHWA, 2010). Because many children travel to school by
bicycle, governments in these countries consider training in safe
cycling essential to improving their safety. Car drivers and truck
drivers sometimes also receive education and awareness programs
in relation to cyclist safety. However, no effects of education on
cyclist safety have been found in scientific research and it is recommended to carry out these studies using, for example, approaches
proposed in the CAST-study (Delhomme et al., 2009).
Another way to reduce risks for cyclists is enforcement. We
found little evidence of specific police campaigns on the behaviour
of cyclists. A second approach is police enforcement related to
driver behaviour, for example by enforcement of speeding and
drinking and driving laws. Better driver behaviour of motorists
can be expected to also reduce risks for cyclists, but no evidence
is available to support this.
4. Sustainable Safety: how to reduce cyclist injuries
drastically?
Because traditional policies were becoming less effective and
less efficient in many highly motorized countries, and because the
inherent risks of our road transport problems had not adequately
been addressed, the Sustainable Safety vision was developed in
the Netherlands (Koornstra et al., 1992; Wegman and Aarts, 2006;
Wegman, 2010). The increasingly diffuse character of the road
safety problems requires a different approach from that in the past.
With the Sustainable Safety vision SWOV believes it has found a
suitable answer. Sustainable Safety has been acknowledged as an
answer suitable for many more, if not all countries worldwide,
as expressed by the World Health Organization, the World Bank
(Peden et al., 2004) and the OECD/ITF (OECD, 2008). The OECD uses
Sustainable Safety and Vision Zero from Sweden as examples of a
Safe System approach.
Road traffic today is inherently dangerous. Other than for railroads and air traffic, the road traffic system is not being designed
with safety as a starting point. The interventions for bicycle safety
that were discussed in the previous section indicate how to avoid
crashes by preventing errors and violations. The principles and
planning methods behind these measures are important. On one
hand, we can adjust the environment to the human measure in
such a way that people commit fewer errors and, consequently,
have a lower risk. On the other hand, it is necessary to deal effectively (and efficiently) with violations and unsafe human behaviour
(excessive/novice behaviour).
In preventing cyclist crashes we are almost fully dependent on
the behaviour of the individual human being. However, human
25
Table 3
The five Sustainable Safety principles.
Sustainable Safety principle
Description
Functionality of roads
Monofunctionality of roads as either
through roads, distributor roads, or access
roads, in a hierarchically structured road
network
Equality in speed, direction, and mass at
medium and high speeds
Road environment and road user
behaviour that support road user
expectations through consistency and
continuity in road design
Injury limitation through a forgiving road
environment and anticipation of road user
behaviour
Ability to assess one’s task capability to
handle the driving task
Homogeneity of mass and/or
speed and direction
Predictability of road course
and road user behaviour by
a recognizable road design
Forgivingness of the
environment and of road
users
State awareness by the road
user
Source: Wegman and Aarts (2006).
beings commit errors and violations. This is the reason why we
need a paradigm shift to a sustainable safe traffic system: we do
not want to hand over a road traffic system to our children in which
approximately the same number of people will be injured or killed
as is the case today.
4.1. Sustainable Safety
The Sustainable Safety vision was launched in 1992 with the
following ambition: “In a sustainably safe road traffic system,
infrastructure design inherently and drastically reduces crash risk.
Should a crash occur, the process that determines crash severity
is conditioned in such a way that severe injury is almost excluded
(Koornstra et al., 1992)”. Initially Sustainable Safety used three principles, functionality, homogeneity and predictability, but the vision
was recently updated and refreshed with new scientific insights
(Wegman and Aarts, 2006).
The updated vision on sustainable safe road traffic (Wegman and
Aarts, 2006) has five central principles: functionality, homogeneity,
predictability, forgivingness and state awareness (Table 3).
The key aspects of the Sustainable Safety approach that have
been identified (Wegman, 2010) are:
• Ethics
◦ We do not want to hand over a traffic system to the next generation with the current fatality and injury levels; these must
be considerably lower.
◦ A proactive approach.
• An integral approach
◦ Integrates man, vehicle and road into one safe traffic system.
◦ Covers the entire road network, all vehicles and all road users.
◦ Integrates road safety with other policy fields.
• Man is the measure of all things
◦ Human capacities and limitations are the guiding factors.
• Reduction of latent errors (system gaps) in the system
◦ In preventing a crash we will not fully be dependent on whether
or not a road user makes a mistake or error.
• Use the criterion of preventable injuries
◦ Which interventions are most effective and cost-effective?
Sustainable Safety identifies three basic factors that play a role
in danger, risk and harm: speed (in crashes), mass/protection (of/by
vehicles) and physical vulnerability (of man). All these factors, and
the five principles, are very relevant for preventing crashes and
reducing injuries of cyclists.
Based on this vision, many road safety measures have
been implemented in the Netherlands and these measures
26
F. Wegman et al. / Accident Analysis and Prevention 44 (2012) 19–29
and investments have been evaluated (Weijermars and van
Schagen, 2009). The conclusion is that a substantial number
of traffic safety measures were implemented in the period
1998–2007, and these measures had a positive road safety effect
(a 32% reduction in traffic fatalities) with a cost-benefit ratio
of 1:3.6.
4.2. Sustainable Safety and cyclists
As illustrated in Section 2 cyclists (and pedestrians) are unprotected in traffic and travel at low speeds and mass. This makes them
vulnerable and they can suffer very severe consequences in crashes
with other road users. Therefore, preventing crashes between fast
and slow traffic is one of the most important requirements for
sustainable safe road use by cyclists (and pedestrians). The measures that need to be sought must be aimed at the ‘disarmament’
of motorized traffic (physical separation and, if that is not possible,
reducing impact speeds). Furthermore, we have to prevent cyclists
from falling off their bikes, for whatever reason, because these falls
can result in severe injuries. These injuries will not always be visible in the official statistics due to underreporting (see Section 2.1).
One of the key aspects of Sustainable Safety is to cover ‘the entire
network, all vehicles and all road users’. As was indicated earlier,
‘high risk groups’ among cyclists (the young and the old) can be
identified, but it will hardly be possible to identify ‘high risk locations’ for cyclists. When they meet motorized traffic travelling at
high speeds, crashes will have serious consequences. Therefore, we
should eliminate this type of conflict everywhere. If that is not feasible, we can slow down motorized traffic to a safe speed. If a collision
then occurs, it will not result in serious injury.
Of course, this approach has far reaching consequences as it may
require much physical space and large investments. But the main
problem seems to be found in society: can a society accept investments in facilities for cyclists given the limited amount of physical
space, given the limited resources, and given the fact that other
problems (congestion relief, environmental problems) also require
investments?
When talking about costs and cost-benefits of investments for
cyclists, it is important to understand that traditional CBA’s and
impact assessments are rather motorized vehicle oriented and
currently do not include all effects (Elvik, 2000). Elvik concluded
that a number of factors that are relevant for cyclists had not
been included (changes in amount of cycling, changes in travel
time for cyclists, changes in feelings of safety and in road users’
health). Some hypothetical examples were discussed by Elvik who
concluded that inclusion of relevant factors for cyclists (and pedestrians) can be decisive factor that would support these investments
to encourage bicycle use and cyclist safety.
Some measures that have been implemented to achieve sustainable safe road traffic and that have positive effects for vulnerable
road users are:
(1) separation of traffic flows that differ in speed, direction and
mass on a systematic basis (separate bicycle tracks alongside
roads);
(2) ‘moped on the carriageway’ instead of mopeds using bicycle
tracks;
(3) construction of 30 and 60 km/h zones;
(4) mandatory side-underrun protection on new heavy goods vehicles;
(5) development of a pedestrian and cyclist-friendly car front.
The first three measures are particularly aimed at preventing
crashes, and the latter two measures aim to reduce the severity of
crashes if they should occur.
5. More cycling and its road safety impact
A growing interest in the promotion of cycling by making it more
attractive can be observed in highly motorized countries. This is
also the case in the plans of many European cities. Signals indicating a move in this direction are also coming from countries like
the United States and Australia. Other than the odd exception like
Bogota in Peru, such signals are not received from Asian, African
and Latin American countries. In contrast, these countries rather
have an increase in car use which results in fewer people using a
bicycle.
But if the numbers of bicycles and cyclists were to increase, relatively dangerous kilometres would be added to present road traffic
as a kilometre travelled by bicycle is more hazardous than a kilometre travelled by car. If a car kilometre were to be replaced by a
bicycle kilometre this could result in a higher casualty rate. Investigating the validity of this premise is interesting. Two approaches
are being taken to do so. In the first place the ‘Safety in Numbers’
theory is being considered. In the second place, the results of scenario studies that have estimated the safety effects of replacing
vehicle kilometres by bicycle kilometres are being evaluated.
For road safety developments it is very interesting to learn the
safety impacts of increased bicycle use. We can approach this question from different angles. The first angle is the phenomenon of
non-linearity of risk: an increase of exposure (numbers, volumes,
etc.) results in a less than proportional increase of the number of
crashes (Eenink et al., 2007). This implies that if the number of vehicles increases, the crash rates will go down. There even is some
evidence that if the number of vehicles grows any further, not only
will the crash rates come down, but also the crash density (crashes
per kilometre of road length) will decrease (Fig. 5).
The risks of cyclists and pedestrians are also non-linear, that is to
say an increase in numbers results in a non-proportional increase
of crashes. Elvik summarized some of these studies (Elvik, 2009).
Most of the studies used by Elvik developed so-called prediction
models of the following form:
NCYC = ␣ · QMV ␤2 · QCYC ␤2
in which NCYC is the number of bicycle crashes, ˛ is a scaling parameter, Q represents volumes of motor vehicles, and QCYC represents
the number of cyclists.
It is of interest to learn the values of the exponents. Elvik concluded that either coefficient takes a value for cyclists between 0.31
and 0.65. That means lower than 1. The same is true for an increase
of the number of motor vehicles (0.46 < ˇ < 0.76). This means that
the risk for each cyclist declines if QCYC increases; the risk for each
driver of colliding with a cyclist declines if QMV increases; and
the risk for each cyclist increases as the number of motor vehicles
increases. This suggests that the total number of crashes could go
down if a substantial share of the journeys by motorized transport
is transferred to cycling.
This non-linearity in risk entered the cycle safety literature in a
study by Jacobsen in 2003, and many authors have referred to this
study since. Jacobsen analyzed several different data sets from the
United States (California) and European countries. He correlated
the measure of injuries to cyclists with the amount of cycling. His
main conclusion is that a driver is less likely to collide with a person
cycling when there are more cyclists (approximately 0.4 power).
Jacobsen also concludes that “Policies that increase the numbers
of people bicycling appear to be an effective route to improving
the safety of people bicycling’. It can be added here that Jacobsen reached the same conclusion for pedestrians. This result has
become widely quoted in bicycle safety circles (Mapes, 2009). But
before adopting this conclusion it is important to investigate the
reasons for this non-linearity of risks.
F. Wegman et al. / Accident Analysis and Prevention 44 (2012) 19–29
27
5,0
Road crashes per kilometre
4,5
4,0
3,5
3,0
2,5
2,0
1,5
1,0
0,5
0,0
0
10000
20000
30000
40000
50000
AADT
Fig. 5. Relationship between traffic volumes (AADT) on Dutch rural roads and accident density (accidents per kilometre road length).
Source: Reurings and Janssen (2006).
From one perspective, there is the explanation of expectancy.
That is to say: if a road user expects the presence of another road
user, or can predict the behaviour of that other road user, one may
expect lower risks (Houtenbos, 2008). Räsänen and Summala carried out an in-depth study of 188 bicycle–car accidents in Finland
studying the actual movements before the crash. They found that
only 11% of the drivers noticed the cyclist before the impact (and
68% of cyclists noticed the car). Under Finnish conditions with
low volumes of cyclists motorists do not seem to expect cyclists.
Another interesting finding is that 92% of the cyclists that had
noticed the car expected right of way (as required by law) but did
not get it. In the words of Hudson (1978) it is not just a matter of
expectation, but also of respect: “the fact that cyclists’ rights are
more respected in towns where cycling is prevalent suggests that
an increase in the number of cyclists on all roads would condition
car drivers to expect and allow for them”. The presence of large
numbers of cyclists may also help underpin their legal use of roadways and intersection crossings and generate public and political
support for more investment in bicycling infrastructure.
In a country, such as the Netherlands, in which cyclists are a
very common feature in everyday life, expectation also plays an
important role. Although generally cyclists are expected, cyclists
coming from an unexpected direction run a higher risk than cyclists
from the expected direction (Schepers and Voorham, 2010)
However, if numbers of cyclists are correlated with risks and
these numbers are assumed to be the only explanation, we are
in error. Large numbers of cyclists in countries such as the
Netherlands, Denmark and Germany are associated with high densities of bicycle facilities. If not both numbers of cyclists and bicycle
facilities are taken into account, the wrong conclusions may be
arrived at. There is no solid evidence that the low fatality rates in
Fig. 1 can only be explained by ‘numbers’. Therefore, Jacobsen’s conclusion may be wrong if we simply add numbers of cyclists to the
system without adding safety quality, that is to say, risk reducing
measures. From this perspective Wegman paraphrased the safety in
numbers by the ‘awareness in numbers’ theory (in Mapes, 2009):
expectancy/awareness is one important factor, but the other one
is safe conditions for cyclists, and it is not evident yet which of
these two—or perhaps another factor—has resulted and will result
in lower risks. When investing in facilities to make cycling safer this
is an important area for further research, in relation to the Adonis
study findings of different phases/stages of development of safety
facilities for cyclists.
From a different perspective researchers have tried to estimate
the safety consequences of a modal shift; more specifically from a
transfer of car kilometres to bicycle kilometres. Elvik (2009) tried
to estimate the number of crashes by adding up the number of
motor vehicle crashes, the number of single vehicle crashes and the
number of bicycle–motor vehicle crashes (in his study he included
pedestrians as well). Elvik proposed six different scenario’s for
change (transferring trips from cars to pedestrians or cyclists, different parameters in the prediction models). Although injury rates
for cyclists are considerably higher than for car drivers, if we accept
the non-linearity of risks (more cyclists, lower risks), the results of
Elvik show that the number of crashes could go down if a substantial
share of trips by motorized transport were transferred to walking
and cycling. How realistic such a shift could be, is debatable.
Stipdonk and Reurings (2010) approached the same question
in a different manner. In their study they substituted only a small
fraction of the short car trips in the Netherlands by bicycle trips, a
substitution of 10% of car trips shorter than 7.5 km, and then they
estimated the number of casualties. It was a ceteris paribus analysis: all relevant parametres are assumed to remain equal. They
distinguished between age groups and gender. Stipdonk and Reurings took into account not only the risks for the car driver or cyclist,
but also the risks run by other road users. From their estimate it can
be concluded that if the young drivers (<35 year) switch to a bicycle the number of fatalities would decrease, but when older drivers
switch, the numbers will increase. The safety effects turn out to
be somewhat more positive if kilometres travelled on roads with
a speed limit of more than 80 km/h (and the risks associated with
this) are excluded (same procedure as in the report by Dekoster
and Schollaert, 1999).
Despite the fact that the risks for bicycles are higher than
the risks for motorized vehicles, the two examples and the nonlinearity of risks combined with the ‘awareness in numbers’
concept lead to the conclusion that when the number of cyclists
increases, the number of fatalities might increase, but this is not
necessarily so and is dependent on conditions. It depends on
parameters of such a substitution (which specific kilometres are
substituted) and on the factual risks of these kilometres for cyclists
and car drivers. There is strong evidence that well-designed bicycle
28
F. Wegman et al. / Accident Analysis and Prevention 44 (2012) 19–29
facilities—physically separated networks—reduce risks for cyclists,
and therefore will have an impact on the net result. Also, the expectation of other road users to have cyclists in their vicinity and to
respond to that, might be an important factor. It is recommended
to explore this relationship further, in order to support policies of
promoting cycling as safely as possible.
6. Discussion and conclusions
1. Cyclists run a relatively high risk of being injured in a road crash.
The most common problem for cyclists worldwide is that our
modern traffic system is designed largely from a car-user perspective. This results in lack of coherent planning and design for
cyclists. Cyclists are vulnerable road users because the human
body is fragile and they lack protection in a crash. Their main
problems are crashes with motorized vehicles. But also singlevehicle crashes are a significant part of their safety problems.
2. Underreporting of road crashes in police registrations is a serious problem especially for cyclists. The official standard crash
statistics, as collected by the police, seem to underestimate
the problem. To tackle underreporting of crash involvement by
cyclists, statistical analyses based on police data need to be complemented by methods such as linking police data with hospital
data, direct observation of events in traffic that are valid proxies
for collisions (traffic conflict techniques), the observation of particular characteristics of traffic behaviour and analysis of their
determinants, and in-depth collision injury research. Comparisons among countries should be carried out with great caution.
3. Relatively little research has been done on the safety of cyclists
compared with that of other road users. Hardly any studies
have investigated why crashes involving cyclists occur and what
the main contributory factors are. The available peer-reviewed
research concentrates on solutions such as the safety effects
of bicycle helmets, roundabouts and cycle tracks. In addition,
knowledge from research cannot easily be generalized, because
local conditions related to cycling and cyclists’ safety differ considerably. This is not so much the case for research into injury
severity where the laws of (bio)mechanics apply everywhere.
Very little research has been published on the safety consequences of policies or strategies aimed at promoting bicycle use
and bicycle safety. This is equally the case for research aimed at
determining the effectiveness and efficiency of interventions in
the areas of planning and design of traffic facilities. Lastly, the
question remains to which extent the available research results
influence decisions to make traffic safer for cyclists.
4. In essence, there are two ways to increase cyclist safety, ways
which fit into the Safe System approach: one is to prevent the
possibility of encounters between cyclists and motorized traffic by giving each group of road users its own network. The
second—in case these unequal transport modes can meet and
a crash can indeed happen—is to reduce the speed of motorized
traffic and introduce vehicle facilities that can reduce the risk
of crashes and their severity. This way the exposure to risk, the
crash rates and the injury rates should be minimized. Measures
to accomplish this mainly involve the stages of planning and
design of traffic facilities. So-called ‘best practice’ information is
available but applying it in different circumstances and under
different conditions than when it was collected, should be done
with careful expert judgments. Safe vehicle design can also make
a contribution. Education and enforcement could possibly also
play a role, but specific policies and related research on cyclist
safety are hardly available.
5. Traditional cost benefit analyses and impact assessments in
transport are rather motorized vehicle oriented and currently
do not include all variables. For example, changes in modal split
(including walking/cycling), changes in travel times for pedestrians and cyclists, change in road user insecurity (feelings of
safety), changes in road user (cyclist) health are typically ignored.
If these variables were included, investments in facilities for
cyclists (and pedestrians) would be considered more beneficial
from a societal point of view.
6. Despite the fact that the risks for cyclists are higher than the risks
for motorized vehicles, it is not necessarily so that an increase in
bicycle kilometres results in an increase in the number of bicycle
casualties. It is dependent on the conditions of this increase. For
example if the increase in bicycle kilometres is a substitute for
(short trip) car kilometres or if the increase is accompanied with
the expansion of well-designed and effective bicycle facilities,
the net safety effects may still be positive. Further exploration
of this relationship is recommended to support policies of promoting cycling as safely as possible.
7. A growing interest can be observed in many highly motorized
countries in promoting bicycling. This is motivated by the anticipation that more cycling will result in positive societal impacts;
especially when more cycling is accompanied by a reduction of
kilometres travelled by motorized vehicles (leading to increased
health, less pollution, and less congestion). However, great care
needs to be given to the design of these policies in order to prevent a worsening of safety; specifically cyclists’ safety. The main
focus in planning and design should be on safe bicycle facilities,
preferably on a systematic basis.
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