Average Reaction Time for Drivers: 0.75 or 1.5 Seconds?

The average reaction time for drivers paying close attention is about 0.75 seconds, but only when they know when and where the signal will come (Green, 2000). A lead car’s brake lights take about 1.25 seconds, and a true surprise about 1.5 seconds. At 50 mph, 1.5 seconds is 110 feet (34 meters) of road before the foot reaches the brake.

Below: why two numbers exist, how far a car travels at 30, 50 and 70 mph, what age changes, and how to measure your own reaction. New here: US road designers assume 2.5 seconds, more than three times the best case. At 50 mph that gap is 128 feet (39 meters), and it is our own arithmetic from two cited values.

Key takeaways

• Expectation sets the number. A driver who knows when and where the signal will come needs about 0.70 to 0.75 seconds. A surprise needs about 1.5 seconds (Green, 2000).
• Reaction time here means detecting the signal and moving the foot from the accelerator to the brake pedal. Braking itself comes after.
• At 50 mph, the reaction distance is 55 feet at 0.75 seconds, 110 feet at 1.5 seconds and 183 feet at the 2.5 seconds used in US road design.
• Experts disagree on the exact values: a 2000 commentary found 1.0 to 1.3 seconds in fairly urgent real-road cases (Summala, 2000).
• Faster reactions go with a higher IQ only weakly, at about -.3 in simple lab tests.

What is the average reaction time for drivers paying close attention?

About 0.70 to 0.75 seconds, if the driver is fully aware of when and where the signal will appear. That is the best-case figure from Marc Green’s 2000 review of driver perception-brake times.

Paying attention alone is not enough. When the signal is unexpected but common, such as a lead car’s brake lights, Green puts the time at about 1.25 seconds. For a surprise, such as an object suddenly moving into the road, the time is roughly 1.5 seconds.

So the answer to the quiz question depends on the situation. If you only know the “3/4 second” rule, you know the best case.

Why do two numbers exist, 0.75 and 1.5 seconds?

Because expectation changes the time by a factor of 2, Green found. The same driver is quick when the signal is expected and slow when it is a surprise.

In perception reaction time driving studies, engineers measure the delay between a signal and the foot reaching the brake. It includes noticing the signal, deciding and moving the foot. It does not include the braking itself.

Not everyone agrees on the middle values. Summala (2000), writing in the same journal issue, argued that Green’s figures mix situations of different urgency. He cited road data in which unalerted drivers braked with an average delay of 1.0 to 1.3 seconds when a collision was about 4 seconds away.

The 2.5 seconds is a different thing: a design value. The Washington State DOT Design Manual says its perception and reaction distance is the distance travelled in 2.5 seconds at the design speed.

How far do you travel during the reaction time at 30, 50 and 70 mph?

At 50 mph a car covers 55 feet (17 meters) in 0.75 seconds and 110 feet (34 meters) in 1.5 seconds. Distance is speed times time, so stopping distance and reaction time grow together.

Reaction distance is only the first part. Then the car has to slow down. The same WSDOT manual calculates design stopping distance with a constant deceleration of 11.2 feet per second squared. Adding our braking arithmetic (speed squared divided by twice that rate) to the 2.5-second reaction gives:

At 50 mph that is about 423 feet in all. Real stopping depends on the road, the tires and the driver, so treat these as examples, not a safety rule.

In the UK, Rule 126 of the Highway Code says to allow at least a two-second gap behind the vehicle in front on high-speed roads, and at least double on wet roads. Two seconds is only half a second more than the surprise reaction time of 1.5 seconds.

Does driver reaction time change with age?

A little, in simple lab tests; and Green notes that age, gender, cognitive load and urgency all modulate the driving times. Reaction time while driving by age is harder to pin down than a button press.

Our reaction time by age article reports that simple reaction time in one calibrated study rose from 218 milliseconds at ages 18 to 24 to 239 at ages 59 to 65, about 0.55 milliseconds a year. That gap of 21 milliseconds is tiny next to the 750 milliseconds that separate an expected signal from a surprise.

A simple click is not a brake. Driving adds perception, a decision and a foot movement. How reaction speed changes across life is one part of the story told in can your IQ change over time.

Do faster reactions mean a higher IQ?

Only weakly. Our reaction time by age article puts the link between reaction time and IQ at about -.3, in simple lab tests. A correlation of -.3 means reaction time accounts for roughly 9% of the differences in IQ.

We did not find a driving study that reports an IQ link, so we make no claim that fast drivers score higher. The nearest related idea is processing speed, covered in processing speed and IQ. For attention under conflict, see the Stroop test explained.

How can you measure your own reaction time?

Use a simple click test. Our reaction time test guide explains how to take one and what the milliseconds mean. It measures a simple response, so it is not a driving test and does not predict how you brake.

To practise attention under conflict, try the Stroop test game or the sequence memory test. More games are on our brain games page. If you are worried about your thinking while driving, talk to a clinician: the CMS list for a detailed cognitive assessment includes a safety evaluation for motor vehicle operation. Our Medicare memory test guide explains the quick screen that often comes first.

What do we still not know about driver reaction time?

• The values come from reviews and commentary, and they disagree at the middle: 1.25 seconds against 1.0 to 1.3.
• The 2.5 seconds is a design value, not a measured average for any one driver.
• Our distance tables are arithmetic from the cited times, not measurements of cars on a road.
• We found no study here that links driver reaction time to IQ.

If you want a number for your thinking skills, take the IQ test. It is built and normed for people, not for driving.

Quick answers

• What is the average reaction time for drivers? About 0.75 seconds if the signal is expected, about 1.25 for a lead car’s brake lights and about 1.5 for a surprise.
• Why 0.75 or 1.5? Expectation. It changes the time by a factor of 2 (Green, 2000).
• How far is that at 50 mph? 55 feet at 0.75 seconds, 110 feet at 1.5 seconds and 183 feet at 2.5 seconds.
• Does reaction time slow with age? Slightly in simple lab tests, about 0.55 milliseconds a year.
• Do faster reactions mean a higher IQ? Only weakly, about -.3 in simple tests.

The bottom line

The average driver needs about 0.75 seconds in the best case and about 1.5 seconds for a surprise, and road designers assume 2.5. Pick the number that matches the situation. These figures describe averages, so any one driver may differ.

Sources

• Green M (2000). “How long does it take to stop?” Methodological analysis of driver perception-brake times. Transportation Human Factors 2(3), 195-216.
• Summala H (2000). Brake reaction times and driver behavior analysis. Transportation Human Factors 2(3), 217-226.
• Washington State Department of Transportation. Design Manual M 22-01, chapter 1260, Sight distance.
• UK Government. The Highway Code: general rules, techniques and advice for all drivers and riders (rules 103 to 158).
• Centers for Medicare & Medicaid Services. Cognitive Assessment & Care Plan Services.