PROCEEDINGS of the HUMAN FACTORS AND ERGONOMICS SOCIETY 37th ANNUAL MEETING-1993
BRAKE PERCEPTION-REACTION TIMES OF OLDER AND YOUNGER DRIVERS
Neil D. Lerner
COMSIS Corporation
Silver Spring, MD
The time drivers require to react in braking situations underlies many practices in highway design and operations.
There is concern whether the perception-reaction time (PRT) values used in current practice adequately meet the
requirements of many older drivers. This study compared on-the-road brake PRTs for unsuspecting drivers in three
age groups: 20-40, 65-69, and 70-plus years old. The method included features to enhance the ecological validity
of the observed reactions: subjects drove their own vehicles in their normal manner; driving was on actual roadways;
extended preliminary driving put the driver at ease and without expectationof unusual events at the time of the braking
incident; the incident occurred at a location lacking features that might enhance alertness (e.g., curves, crests,
driveways). Subjects drove an extended route, under the guise that they were making periodic judgments about "road
quality." At one point, a large crash barrel was remotely released from behind brush on a berm and rolled toward
the driver's path. Although most of the fastest observed PRTs were from the young group, there were no differences
in central tendency (mean = 1.5 s) or upper percentile values (85th percentile = 1.9 s) among the age groups.
Furthermore, the current highway design value of 2.5 seconds for brake PRT appears adequate to cover the full range
of drivers.
values are only loosely linked to realistic driving data, to
the extent they are empirically based, the underlying
research has not appropriatelyconsidered elderlydrivers.
The general slowing of responses with age is a broad and
well establishedlaboratory finding (e.g., Salthouse, 1985),
including studies showing slower foot pedal reaction times
for older subjects(e.g., West VirginiaUniversity, 1988).
To the extent that older drivers require longer PRTs than
those established for highway design pmtice, they may be
at a seriousdisadvantagefor numerous driving situations.
For safety reasons, it is important that design and
operational practica meet the needs of all segments of the
drivingpopulation. However, increasing the assumed PRT
value for highway design would have extensive
ramifications, and substantial costs.
INTRODUCTION
Driver perception-reaction time (PRT) is a
fundamental concept in highway design and safety.
PRT refers to the time required to perceive, interpret,
decide, and initiate a response to some stimulus.
Different driving situations are characterized by
different assumed values of PRT. For example, the
PRT associated with reacting to an obstacle in the
vehicle's path (brake reaction time) is different from
the PRT associated with determining that it is safe to
begin moving through an intersection, or with the time
required to judge whether it is safe to initiate a passing
maneuver. One of the most basic concepts for
highway design and operations is that of stopping sight
distance (SSD). SSD is the distance traveled before
coming to a stop when a driver, travelling at roadway
design speed, must brake in reaction to an unexpected
obstacle in his or her path. Driver PRT is a key
determinant of SSD, so that the specific PRT value
assumed for design purposes influences many aspects
of the roadway. Currently, the assumed value for
design equations is 2.5 s (AASHTO, 1990). This
parameter ultimately influences such roadway features
as horizontal curve radius, vertical curves (crests),
approaches to intersections, sign placement, traffic
signal visibility and phasing, and other common
roadway aspects.
Although PRT generally slows with age, there are
some reasons for presuming that current sight distance
criteria may be adequate for older drivers and do not
require change. Lerner (1991) has discussed a number of
these. Among the more interesting is the finding that the
few studies that have measured on-the-road PRTs for
unsuspecting drivers of different ages have not obtained
meaningful differences between age groups (Olson,
Cleveland, Fancher, Kostyniuk, and Schneider, 1984;
Korteling, 1990; Hostetter, McGee, Crowley, Sequin, and
Dauber, 1986). Furthermore, even if older drivers require
somewhatlonger response times, the "cushion"built into
current design parameters may be adequateto cover their
behavior.
Recently, there has been a good deal of concern
about whether the assumed PRT values in design
equations are adequate to meet the requirements of
older drivers (Lerner, 1991). Although assumed
The purpose of the present study, funded by the
Federal Highway Administmtion,was to measure realistic,
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PROCEEDINGS of the HUMAN FACTORS A N D ERGONOMICS SOCIETY 37th ANNUAL MEETING-1993
instructedthat we had permission to continueon this road
and drove around the barricades, and were told the
appropriate speed was 40 mph. When the vehicle reached
a location near the midpoint of the roadway section, a large
yellow crash barrel, hidden on a berm behind some brush,
was remotely released and suddenly became visible r o h g
toward the roadway. Although it appeared to be rolling
directly into the road, a set of chains held the barrel to the
shoulder area.
The barrel emerged into view
approximately 200 feet in front of the vehicle; this
provided a time-to-collision of about 3.4 s at the target
speed of 40 mph.
on-the-road braking PRTs for unsuspecting older and
younger drivers, and to determine whether the
currently assumed design value of 2.5 s is adequate for
drivers of all ages. The study was designed to provide
ecologically meaningful data, avoiding some of the
limitations of past research. Additional detail on
methods and analysis can be found in a project final
report to the Federal Highway Administration (Lerner,
Huey, McGee, and Sullivan, in preparation).
METHOD
The purpose of the study was to elicit and
measure realistic brake reaction times from drivers.
Given this, several methodological features were
deemed important. The subjects should be driving on
actual roadways. They should be driving normally and
at ease. They should have no expectation of an
emergency braking event and should not be leery of
the motives of the experimenter. They should be
driving their own vehicles, rather than trying to adapt
to an experimenter-provided vehicle. This was of
particular concern for older drivers, who may have
selected or adapted their vehicles to meet their needs,
and who may not adapt to a test vehicle as quickly.
The data were recorded using a video-based data
collection system. For subjects with the first procedure,
the emergenceof the barrel was recorded by an in-vehicle
camera, and the occurrence of the brake response by
activation of a pressuresensitivetape switch attached to the
brake pedal. In the other procedure, a hidden roadside
microcamemrecorded both the emergence of the h l and
the activationof the vehicle's brake lamps. The PRT was
the interval between the emergence of the barrel and the
initiation of braking.
All sessions were run in daytimeand clear weather.
Subjects were recruited in three age groups: 20-40 years
old, 65-69 years old, and 70 or older. To minimize the
selection bias toward more capable elderly, older subjects
were recruited in the greater Washington, DC area,
working through senior centers, churches, retirement
communities,and so forth. Rather than placing initiative
on the subject to volunteer, as much as possible we worked
with directors of the institutions to help identify and
approach individualswith a wide range of capabiities, and
to provide social support and incentive for taking part.
Although there can be no claim that the sample was
representative, and while it is likely that those at the
extreme lowest limits of ability and confidencetended to
exclude themselves, the older group did appear to provide
a broadly suitable range, and certainly included many
individuals who would have been unlikely to participate
without more active recruiting strategies.
In order to meet these methodological
requirements, subjects drove their own vehicles over
an extended route, under the guise that they were
participating in a study that was recording their
judgments of "road quality." Periodically (at stop
signs), they made judgments about the quality of the
road sections they had just travelled. Two slightly
different procedures were involved. For some
subjects, the initial part of the drive was incorporated
with another experiment, and the subject had been
driving about an hour prior to encountering the braking
event. For the other subjects, the prior part of the
drive was briefer, covering about three miles. (Brake
reaction times for the two groups were not significantly
different, t=0.08, and the datasets were combined for
analysis). It was only at the completion of the route
that the subjects encountered the site of the braking
event; from their perspectives, they were simply
continuing the ride, and the procedure, that had been
in effect all along. They were driving in a relatively
relaxed, normal manner, with no expectation of any
unusual event (post-session debriefings confirmed this).
The subjects turned onto a four lane divided highway.
This highway provides access to an interstate highway,
and then continues on for 0.7 miles beyond the
freeway entrance. This extended stub of roadway is a
functional, fully delineated roadway, but is closed to
normal traffic by the use of barricades. Subjects were
Although over 200 subjects participated in the study,
there was a very high rate of data loss due to a
combination of factors, includingequipmentfailures,video
problems, weather, experimental error, inappropriate
subject behavior, unauthorized traffic at the site, and so
forth. Valid trials and records were obtained for 116
subjects; this included 30 in the 20-40 year old group, and
43 in each of the two older groups.
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PROCEEDINGS of the HUMAN FACTORS A N D ERGONOMICS SOCIETY 37th ANNUAL MEETING-1993
automatic transmissions, while only about two-thirdsof the
younger group did. However, post hoc comparisons
indicatedthat transmission typehad no discernibleeffect on
either the nature of the reaction to the barrel (brake, steer)
or the speed of braking.
RESULTS
Nearly all (87%) of the 116 drivers made some
overt vehicle maneuver in response to the emergence
of the barrel. Of these, about 43% both steered and
braked, 36% steered only, and 8% braked only. Thus,
just over half of the drivers (51%) reacted by braking.
This is consistent with various other on-the-road
studies, which have found steering to be a more
reliable reaction, and with the percentage of braking
drivers varying from about 30% to about 80% (e.g.,
Triggs and Harris, 1982). Measurable brake reaction
times were obtained for 56 subjects; this included 14
of the 20-40 year olds, 18 of the 65-69 year olds, and
24 of those 70 or older.
DISCUSSION
This study provided uniquedata on the brake PRTs of
drivers of different ages. It used real drivers, of known
ages, in their own vehicles, driving on actual roadways,
under conditions where they were not expecting any
unusual (emergency braking) event. None of the previous
research has met all of these criteria, which are important
for deriving "absolute" measures of brake reaction time
that are ecologically valid.
The mean brake PRT for all subjects was 1.5 s,
with a standard deviation of 0.4 s. An analysis of
variance revealed no significant main effect of age or
gender, with the interaction of age with gender of
borderline significance (p =0.055). The interaction
reflects the particularly short mean reaction time of the
young female group (1.22 s), while other age-bygender group means ranged from 1.40 to 1.65 s. The
age groups not only showed little difference in central
tendency, but also in terms of upper percentile values.
The 85th percentile PRT (a level often used in
developing highway design values) was about 1.9 s for
all age groups. Virtually all responses were captured
by the 2.5 s design value; the longest observed time
was 2.54 s and the next longest was 2.39 s.
consistent with some earlier on-the-d studia, there
was no indication of meaningfullyslower braking by older
p u p s , as measured by central tendency (mean, median) or
upper percentile values (e.g., 85th percentile). Young
drivers responded quickly more often, but also showed a
higher proportion of slow reaction times. There may be a
variety of explanationsfor this. Furthermore, this study
only measured the time to initiate braking, and did not
measure the braking profile (decelerationrate) or degree of
driver control. The age groups could differ in these
regards. However, based strictly on PRT, older groups
were not slower to react. The original research plan
anticipated sufficient data so that reliable frequency
distributions could be obtained for each age group.
However, due to data loss and the fact that only about half
the drivers in such experiments respond by braking, actual
brake reaction times were measured for only 56 of the
original participants. Therefore, a great deal of precision
should not be attached to the observed values.
Nonetheless, this sample is sufficient to indicate that there
is no important difference in the response times of the
various age groups.
However, the absence of differences in central
tendency or 85th percentile values does not necessarily
imply that older and younger groups were responding
in the same manner. Most of the fast reaction times
(e.g., < 1.25 s) were provided by the young drivers.
However, the distribution of reaction times for young
drivers was bi-modal. Most were less than 1.45 s, but
a few were quite long (over 1.9 s). In contrast, for the
older groups, about half of the cases fell in the roughly
half-second interval between these values. Given the
relatively small number of observations for the young
age group, the differences between these distributions
must be viewed with caution; a chi square test
approached, but did not reach, conventional statistical
significance levels (Chi square = 11.2, 6 df;
0.05 <p<O. 10).
The 2.5 s value used for PRT in highway design
applicationsappears to provide adequatecoveragefor the
full range of driver age. The longest response time
recorded was at this value. The mean was a full second
faster, with 2.5 s being more than two standard deviations
slower, and the estimate of the 85th percentilebeing more
than a half-second faster than this value. The findings of
this research are consistent with a number of other on-road
studiesthat have observed brake or steeringresponseupper
percentile (e.g., 85th percentile) times of less than 2 s
(e.g., Olson et al., 1984;Triggs and Harris, 1982;Sivak,
Olson, and Farmer, 1982; Sivak, Post, Olson, and
Donohue, 1981; Allen Corporation, 1978; Summala,
1981). There have been some studies that have observed
Because subjects drove their own vehicles, it is
possible that differences (or the absence of differences)
between age groups could be attributableto differences
in the vehicles they drive. The major difference noted
was that almost all of the older driver's vehicles had
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longer brake response times, but these have not been
for situationsappropriate to SSD. For example, longer
times may be observed in response to stimuli such as
highway signs or changes in signal phase, or where
there are more difficult maneuver requirements related
to complex roadway geometries (treated as "decision
sight distance" situations in traffic engineering terms).
Some reviewers have failed to discriminate these
different situations. There have also been calls to
increase the PRT design value beyond 2.5 s in order to
compensate for the effects of factors such as age,
fatigue, or impairment. Based on typical research
findings to date, the 2.5 s value already provides some
design cushion (of over 0.5 s); it is not clear what the
empirical basis for extending the design value further
is. While any lengthening of the design value might
provide some additional marginal increment in
protection, it would necessitate ubiquitous changes,
with substantial costs involved, throughout the highway
system. Thus the safety benefits of increasing the
assumed PRT for SSD beyond 2.5 s should be
demonstrably significant.
The absence of substantially slower brake PRTs
among older groups provides an illustration of how the
factorsof expertiseand compensation in complex skills can
maintain performance even in the face of reduced
capabilities. Virtually all of the componentpsychomotor
processesthat underlie PRT -- informationpromsing rate,
visual search time, responseinitiation, movement time, etc.
- have been shown to slow with age, in laboratory studies.
Age-relatedcompensation is poorly understood for driving,
as it is for various other skilled tasks. Furthermore, the
mechanisms involved in compensation for one aspect of
performance might be related to degradation in other
aspectsof performance (e.g., braking may be more "all-ornone," providing a greater risk of rear end collisions or
loss of vehiclecontrol). Onepossible explanation for the
absence of a differencebetween age groups is that older
drivers might be responding in a more reflexive,
stereotyped manner. Younger drivers may be prone to do
more evaluation before responding, or respond in a more
gradual or controlled manner, using their faster information
processing capabilitiesto refine the response, rather than
quicken it. It was the subjective opinion of the primary
research assistant who accompanied the drivers that the
older drivers tended to make more evident and dramatic
foot movementswhile braking, although he did not notice
a subjective sense of more severe deceleration. This
observation is also consistentwith the data of Olson et al.
(1984), whose instrumentationallowed the total PRT to be
segmented into a "perception time" (from first target
visibility to release of the accelerator) and a "response
time" (from release of the accelerator to stepping on the
brake pedal). Although there was little differencebetween
age groups in the total PRT, the older group actually had
faster "responsetimes" (estimatingfrom figures, about 0.1
s faster at the 50th percentile and about 0.2 s faster at the
90th percentile). Thus based on our observationsand the
Olson et al. findings, it may be that in a situation where
there is a surprise need for possiblebraking, older drivers
are more consistent in making a rapid move to the brake
pedal, once the hazard has been recognized. The response
may be more stereotyped, and subject to less evaluation
and modulation. Whatever the reason for the absence of
observed differences in overall brake PRT between age
groups for SSD situations, it is none the less clear that
most older drivers can continue to react with appropriate
swiftness, even to an unanticipated braking event. It
should be noted, however, that where the stimulusevents
and required driving maneuvers are more complex and
ambiguous than emergency braking, there might bemore
deleterious effects of age.
There are some reasons why the present
procedure may arguably have led to relatively
conservative estimates of PRT. An analysis of the
brake reaction times of drivers who also showed
substantial steering in reaction to the barrel, compared
to those that braked only, showed that those who
steered had somewhat slower brake PRTs (about a
quarter-second slower). Thus by providing an
opportunity for avoidance steering, as well as braking,
the estimate of brake reaction time may be somewhat
longer than for a high-emergency situation in which
braking is the only clear alternative. Furthermore, the
"hazard" emerged from a hidden location off the side
of the road, rather than being in more central view.
Previous studies may also have used situations in
which there was greater expectancy of the potential
need for braking, either because of a geometric feature
(coming over the crest of a hill, where sight distance
was obscured) or because of the need to monitor
leading traffic (where the stimulus to brake was the
illumination of the brake lamps of a vehicle
immediately ahead), In the present experiment,
subjects were simply driving on a straight section of
roadway, so that the occurrence of any conflict was
quite unexpected. Whatever the reason, this study
observed mean times that were somewhat slower (by
about 0.15 to 0.35 s) than those reported in other onroad studies, such as those cited above. Despite this,
the 2.5 s PRT design value covered the range of
observed brake times.
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