Charles C. Roberts, Jr.
Response time is the total time it takes for a vehicle driver to perceive, evaluate,
decide and react to a situation on the roadway. Since vehicle driver response time
can be as long as 2-4 seconds, highway designers, accident reconstructionists and
the courts take this into account. A highway designer allows several seconds of
unobstructed view of a traffic control sign, giving time for the motorist to respond
to the sign information. An accident reconstructionist utilizes response time to
explain why an accident occurred, i.e., a bicyclist suddenly pulled in front of an
automobile, giving insufficient time for the motorist to respond and avoid the
accident. The courts are interested in apportioning liability to various parties in a
legal action based on response time. At issue, of course, is the question, "Was
there sufficient time to avoid an accident?" Claims professionals deal with this
issue on a daily basis. Claimants often argue that the insured caused an accident
by giving little time to respond. Other losses involving personal injury and
property may be a consequence of a vehicle traveling at a high rate of speed
leaving insufficient time to respond to road situations.
Like many other human characteristics, response time is a highly variable
quantity. For the same road hazard, the response time of vehicle drivers varies
markedly as a function of age, time of day, weather conditions, chemical
ingestion, and fatigue. For a given driver, response times vary depending on type
WHAT IS RESPONSE TIME
The studies of psychology and ergonomics have delved into the intricacies of how
a person responds to stimuli. Current thought considers response time to be a
summation of times required to activate biological functions. For instance, if one
views a hazard on a roadway, there is a time required for conversion of the optical
image into a nerve impulse, time to transmit a signal along a nerve to the cerebral
cortex, time for processing of the signal by the brain, time for transmission of a
signal along a nerve to musculature and time delay of muscle response. The sum
total of these times is often called the response time, a term often confused with
Figure 1 shows a driver sitting at a test apparatus that evaluates reaction time. As
soon as the light turns red on the console, the driver releases the accelerator and
applies the brake. The reaction time is measured. This form of testing is often
called simple reaction time, as it is a result of a single stimulus, the red light.
Reaction times are typically on the order of 3/4 of a second. However, response
times are more complex and can be as high as 3-4 seconds. So what makes up the
difference? The answer is the perception/decision time. The following equation
shows the components of response time:
Response time = Perception/Decision Time + Reaction Time
The perception/decision time is the time it takes to view a hazard and figure out
what to do about it. The reaction time is the time it takes to perform a particular
function once a decision has been made. The response time for removing one's
hand from a hot skillet is relatively quick and is on the order of about a half
second. In this example, a natural response to excessive heat bypasses the visual
sensors, allowing for a quicker response time. Driving an automobile requires a
high degree of visual processing, which tends to extend response times.
McGee et. al. (1) reported that perception time is the sum of eye movement time,
fixation on the hazard time delay, recognition time delay and muscle response
delay time. They found that for the 85th percentile of drivers, eye movement delay
was 0.09 seconds, fixation delay time was 0.20 seconds, recognition delay time
was 0.50 seconds, decision time 0.85 seconds, muscle response delay was 0.31
seconds and brake reaction time was 1.24 seconds. The sum total of these times,
the response time, was 3.19 seconds. The 85th percentile is often chosen as the
upper bound for design analyses.
Many such studies assume that each of the component times are additive to form
the total response time. Some investigators report that in many situations, parallel
processing occurs, indicating that some of the component activities may be
occurring simultaneously, reducing the calculated response time. Also, the type of
hazard being recognized plays a major role in response time. For instance many
laboratory tests involve a person sitting at a vehicle simulator waiting for a light
to signal when to press the brake pedal. This type of testing usually results in
short response times since the participants know what to expect and are ready.
Contrast this with a driver cresting a hill and viewing a semi-tractor trailer rig
broken down in the right lane of traffic, in a no parking area without signals or
markers deployed. At the time of initial visual fixation, it may not be obvious that
the vehicle is not moving, which may extend the perception/decision time
USAGE OF RESPONSE TIME
Figure 2 is a view of an accident scene showing long tire marks and an impact
point at an intersection. Vehicle number 2 had driven across the roadway and was
struck by vehicle number 1. The tire marks of vehicle number 1 prior to impact
measured 234 feet in length. The distance from the crest of the hill was 500 feet.
The speed limit was 45 MPH. If vehicle number 1 had been traveling the speed
limit (45 MPH, 66 ft/sec), it would have taken approximately 7 seconds to reach
the stalled truck, if no braking occurred. Being a clear day with ideal road
conditions, a response time of 3 seconds would appear attainable. Consequently
as vehicle number 1 crested the hill (assuming a 45 MPH speed), approximately
198 feet would be traveled before active braking occurred, with 302 feet left in
which to stop. At a speed of 45 MPH, assuming a drag coefficient of 0.6 (a typical
value for dry asphalt), the vehicle could be brought to a stop in approximately 114
feet, which is well within the 302 feet available. Therefore, if the vehicle had been
traveling at the speed limit, the accident would have been avoidable. The 234 feet
of tire mark suggests that the vehicle was traveling at a minimum of 64 MPH (94
ft/sec) at the beginning of the tire mark (again using a drag coefficient of 0.6).
Assuming a response time of 3 seconds and a speed of 64 MPH, approximately
282 feet were traveled by vehicle number 1 before brake application occurred.
The remaining distance of 218 feet was not sufficient to stop before contacting the
stalled truck. Consequently, if the driver of vehicle number 1 had been traveling
the speed limit, the accident probably would not have occurred. The stalled truck,
crossing the intersection, is an obvious hazard and it can easily be seen that the
vehicle is not moving.
Figure 3 shows a slightly different scenario. A tractor trailer rig has broken down
on a two lane highway with no markers or emergency lighting activated. The
speed limit is 45 MPH. The distance from the crest of the hill to vehicle number 1
is 300 feet. From the rear, it is not obvious that the tractor trail is stalled since no
markers or warnings are deployed. Using a response time of 3 seconds, vehicle
number 1 would have traveled approximately 198 feet (at 45 MPH) before brake
application leaving 102 feet remaining for stopping. The stopping distance at 45
MPH is approximately 114 feet, not enough distance to stop without impact with
the truck. One may argue that a 3 second response time is excessive and that
perhaps a 1.5 second response time is more realistic. However, the one
complicating factor in this accident scenario is the non-obvious nature of the
parked tractor trailer rig. Without warnings deployed, there may be difficulty in
the determination of whether the vehicle is moving or not. This can significantly
extend the perception time, such that a 3 second or higher response time would be
OTHER INFLUENCES AFFECTING RESPONSE TIME
A little more skill is required to operate a motorcycle when compared to an
automobile, consequently the separate licensing requirements in several states. In
sudden hazardous situations, "slamming" on the brakes can have a detrimental
effect, often causing the motorcycle to fall to one side or causing the operator to
be catapulted over the handlebars. Consequently, application of the rear brake
only is a prudent means of avoiding an accident when driving a motorcycle. The
decision to use rear brakes only can increase response time. Also with only the
rear brake being activated, the stopping distance increases since about 50%
braking is being applied. What may be an appropriate response time and stopping
distance for an automobile may not be appropriate for a motorcycle.
It has been well documented that visual functions decrease in proportion to
decreasing illumination. Dirt on headlamps and crazing of windshields over time
also aversely affect visual acuity. Headlight alignment greatly affects the
perception of pedestrians as well as other vehicles.
A person's age affects response time with increases being most notable in the later
years. An example of an age effect is the need for reading glasses or bifocal
lenses. The growing inflexibility of the lenses in a person's eye causes this
condition (presbyopia). Visual acuity typically peaks at about age 15 and declines
to about 33% of the highest value at age 80 (Reference 2). The elderly have been
found to be much more adversely affected due to their loss of acuity and other
visual functions. Typical studies show reactions times of drivers near the age of
70 increase by approximately 20% over those for age 20 (Reference 3).
Chemical usage has been shown to have a substantial effect on response time.
Reference 4 reports that at a blood alcohol content of 0.02% by weight, the
average increase in errors in simulated driving is approximately 6 in the particular
study. At a blood alcohol content of 0.08%, the average increase in errors in
simulated driving is approximately 25 in this particular test. Therefore, the
simulated driving error rate at 0.08% blood alcohol content was 4 times that at
0.02%, indicating that, despite the fact that a driver may have a blood alcohol
level below a legal limit, there is still an effect on the response time.
Reference 5 reports that the average reaction time for women was approximately
15% longer than for men.
What can be gleaned from the previous discussion is that response time is a
distributed quantity because of variability in people, as well as in situations that
require a response. The accident reconstruction community often assumes a
maximum 2.5 - 3.0 second response time. This may be applicable for most
accidents with obvious hazards. Other accidents involving less defined or
confusing hazards may result in longer response times. Other factors extending
response time are age, time of day, gender and chemical usage, suggesting that
response time is typically characteristic of a particular set of circumstances
encountered in an accident.
1. McGee, et. al., Highway Design and Operation Standards Affected by Driver
Characteristics, Volume II: Final Technical Report, Bellomo-McGee, Inc.
Vienna, Virginia, Report No. FHWA-RD-83-015, 1983.
2. Verriest, G., L'influence de l'age sur les fonctions visuelles de l'homme,
Bulletin de L'Academe Royale de Medecine Bellique, 1971, 11, 527-578.
3. American Automobile Association, Traffic Engineering Safety Department,
Age and Complex Reaction Time, Report No 41, 1952.
4. Barzelay and Lacy, Scientific Automobile Accident Reconstruction, Matthew
Bender, New York, 1985.
5. American Automobile Association, Traffic Engineering Safety Department,
Reaction Time as Related to Age, Report No. 69, 1966.
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