What do safe speeds really mean
for New Zealand?
What are safe speeds?
Isn’t it obvious?
Safe speeds are slower speeds
Yes, if you’re familiar with the Power Model
The Power Model
Nilsson (2004)
Elvik, Christensen & Amundsen(2004)
Samuel G. Charlton
Traffic & Road Safety Research Group
Waikato University
But it isn’t obvious to everyone
In fact, there is a vocal segment that argues that
faster is safer, and that it actually reduces risk
Belief that travelling faster than the
surrounding traffic is safer
Solomon’s (1964) “Accidents on main rural
highways related to speed driver & vehicle”
But...
Solomon’s data were from 1950-60s era roads,
which lacked turn lanes and passing lanes
many crashes occurred at intersections and
involved stopped or slowing vehicles
So slower = safer may be too simplistic
…roads with a larger speed variance (lower
homogeneity) had a higher crash rate than
roads with a smaller speed variance.
Kinetic energy released in a crash is
proportional to the square of the
speed of impact
Probability of fatality (for typical
crash speeds) is the fourth power of
speed difference at impact
“Faster is safer” myth disproven
In conclusion, it can be stated that both the older and
the more recent studies provide evidence that driving faster
than the surrounding traffic increases the risk of a crash. With
regard to driving slower than average, the evidence is less
conclusive.
Aarts & van Schagen (2006)
“Faster is safer” myth disproven
but attitudes remain
UK: Insurance brokers’ survey
2011 survey of 4,404 drivers
#1 source of driver frustration?
A slow driver ahead of you
The case for homogeneous speeds
Speed variation is significantly predictive of fatality rates for rural
interstate highways and rural and urban arterial routes; the greater
the speed variation, the higher the fatality rate
Lave (1985)
Aarts & van Schagen (2006)
Reduction of speeding and increase of homogeneity
are known to lower the risk and severity of traffic
accidents... Homogeneity of driving speeds is an
important variable in determining road safety; more
homogeneous driving speeds increase road safety
van Nes, Brandenburg, & Twisk (2010)
So, are safe speeds homogeneous speeds?
3 Sustainable Safety principles
Functionality – a hierarchically structured road network with
clear differences between levels
Homogeneity – equity in speed, direction and mass of
vehicles (at medium & high speeds)
Predictability – supporting road user expectations users
through consistency and continuity of design
(relative to the speed of other motorists)
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In recent years we have had some success
in reducing mean speeds,
but we have not reduced speed variability
In spite of the fact that speed variability
dramatically decreases traffic flows
a highway can move more vehicles per
hour at 80 km/h than 100 km/h
Herty & Illner (2007); Lipshtat (2009)
We don’t like drivers going slower than us,
and we don’t like drivers going faster than us
What produces homogeneous speeds?
Credible speed limits contribute to road safety as increased
credibility leads to better speed limit compliance and is
likely to improve homogeneity of traffic flows.
The credibility of a speed limit is influenced by identifiable
features of the road and its surroundings… This makes it
possible to make a limit more credible… this can be done
by either fitting the limit better to the features, or fitting the
features better to the limit.
Goldenbeld and Van Schagen (2007)
We have a “set point” or threshold for speed tolerance
(our tolerance of other drivers’ speeds)
But we encounter a range of speeds well in
excess of our set point
What produces homogeneous speeds?
(and credible speed limits?)
So, are safe speeds those that are
appropriate to the infrastructure?
Speed curves suggest some contextually safe speeds
Correspondence between the design speed and
the enforced speed limit
The difference between design speed and speed limit plays a role…
Average speed mainly corresponded to the design speed … the crash
rate and speed variance were lowest when the speed limit was (no
more than )5-10 mph (≈8-16 km/h) lower than the design speed.
How do we achieve contextually
appropriate (& safe) speeds?
Garber and Gadiraju (1989)
The look and feel of the road (infrastructure)
Road widths, delineation, and road furniture
explicitly and implicitly afford speed and lane position choices
Sign neglect
Problems of sign neglect,
enforcement credibility
& legacy issues
Drivers pay little attention to signs
Legacy issues & public
resistance
Drivers may notice as few as 1 in 10 road signs
and have very poor memory for road signs
they’ve just passed (6% recall - 9% recognition)
In 1974, hoping to reduce fuel consumption, the US Congress
set a national speed limit of 55 mph
Gets worse with increasing familiarity
The mileage-based fatality rate decreased by 34% in one year
Enforcement credibility
Enforcement can be very unpopular if the speed
limits don’t match the perceived safe speed
Simply changing the speed limit is not credible
“Arbitrary” reductions in speed limits will
increase heterogeneity of speeds
The public did not accept the change
even after 21 years
in spite of the cost to human life
Resist the temptation to simply lower
speed limits and rely on enforcement
Difficult to find the political will to defend this sort change
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So let’s use public education to
change expectations!
What else can we do?
Let’s wait for IVHS / AHS
they are just around the corner
Maybe, but our vehicle fleet changes slowly
Hard hitting road safety campaigns
explaining the need for slower speeds
What else can we do?
Correspondence between the design speed and
the enforced speed limit
We’ve been trying for over 50 years,
with little success
Resist the temptation to simply lower
speed limits and rely on education
The look and feel of a thing
determines how it is used
Correspondence between the design speed and
the enforced speed limit and the road use
Safe speeds are contextually appropriate
to function and infrastructure
What’s been done elsewhere?
Affordances
perceptual properties that function as “built-in” instructions,
“low cost designs that are expected to reduce the number of
traffic casualties by 10–20% ...implemented as much as
possible in an area oriented way”
Jaarsmaa, Louwerseb, Dijkstrab, de Vriesa, & Spaasc (2011)
How do we get drivers to drive at safe speeds?
Multiple cues are better... Redintegration
Speed is “chosen” by drivers moment to moment
A speeding ticket is low probability event
(perceived as unlucky and unfair)
Education campaigns are out of context and
do not guarantee behaviour change
How do we communicate
safe speeds to drivers?
Drivers need cues they can use
What do drivers notice?
We can build better drivers by
designing better roads
#1 change detected by drivers – removal of centrelines
#2 – removal of edgelines
They even detect arbitrary dots on the road
We can build better drivers by
designing better roads
What are safe speeds?
Visually differentiate the levels of
the road hierarchy
A few speed categories are
better than many 30 50 70
Safe speeds are slower speeds
Safe speeds are contextually
appropriate, homogeneous speeds
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Multiple reliable cues are better than a few signs
Improves enforcement – self-enforcing
Speeds that encourage drivers to “Go With The Flow”
A Safe System
approach
Improves education – continuous, self-explaining,
only way to reach some people
Improves economics – safe, homogenous speeds
Traffic & Road Safety Research Group
Waikato University
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