How long is the 2.5-second road design figure?

Typically 2.50 s, with a real range of 2.50 s. The perception-reaction time assumed when calculating stopping sight distance for road geometry.

The 2.5-second road design figure

2.50 s

Design value — chosen to be conservative, not measured

Every stopping-sight-distance calculation in American road design uses 2.5 seconds, and it is not a measurement of anything. It was chosen to cover nearly all drivers under nearly all conditions, including tired, elderly and distracted ones, because a road designed for the average driver kills the slow half. Quoting it as "average driver reaction time" — which happens constantly, including in driving-school material — overstates the real figure by a factor of about three and makes people think they are unusually quick.

The numbers

Typical: 2.50 s
Range: 2.50 s
What it is: Design value — chosen to be conservative, not measured
What is measured: The perception-reaction time assumed when calculating stopping sight distance for road geometry.

Why this is a design value

A design value is chosen to cover nearly everyone, including slow, tired and distracted people, because the cost of underestimating is severe. It is deliberately much longer than a typical measurement and should never be quoted as one.

Where it sits against everything else

Reflexes at the top, reactions in the middle, design values off the end of the scale.

Vestibulo-ocular reflex — Reflex 10 ms

Knee-jerk (patellar) reflex — Reflex 22 ms

Latency added by your screen and mouse — Physical limit 35 ms

Blink reflex — Reflex 40 ms

Withdrawal reflex — Reflex 90 ms

Time a jab takes to land — Physical limit 110 ms

Sprint start reaction — Reaction 140 ms

Simple tactile reaction time — Reaction 155 ms

Simple auditory reaction time — Reaction 170 ms

Professional gamer reaction time — Reaction 190 ms

Formula One start reaction — Reaction 200 ms

Simple visual reaction time — Reaction 250 ms

How reaction time changes with age — Reaction 250 ms

What sleep loss does to reaction time — Reaction 300 ms

What alcohol does to reaction time — Reaction 320 ms

Recognition reaction time — Reaction 340 ms

Choice reaction time — Reaction 380 ms

Time to hit a 95 mph fastball — Physical limit 400 ms

Time a goalkeeper has on a penalty — Physical limit 450 ms

Time to play a 90 mph delivery — Physical limit 500 ms

Driver reaction to an expected event — Reaction 850 ms

The 2.5-second road design figure — Design value 2.50 s

Driving

What	Typical	Range	Kind
Driver reaction to an expected event	850 ms	700 ms–1 s	Reaction
Driver reaction to a surprise	1.35 s	1.10 s–1.80 s	Reaction

Common questions

How long is the 2.5-second road design figure?: Typically 2.50 s, with a real range of 2.50 s. The perception-reaction time assumed when calculating stopping sight distance for road geometry.
Is that a reflex or a reaction?: Design value — chosen to be conservative, not measured. A design value is chosen to cover nearly everyone, including slow, tired and distracted people, because the cost of underestimating is severe. It is deliberately much longer than a typical measurement and should never be quoted as one.

Driver reaction to an expected event 850 ms
Driver reaction to a surprise 1.35 s
A reflex is not a reaction Why one is 20 ms and the other 250
Stopping distance at every speed Reaction distance plus braking distance, computed
Measure your own reaction time Five attempts, median reported

Sources

A Policy on Geometric Design of Highways and Streets (the Green Book) — American Association of State Highway and Transportation Officials

How these figures are compiled and checked