---
title: "The 2.5-second road design figure: 2.50 s"
description: "The 2.5-second road design figure: typically 2.50 s, really 2.50 s. What is measured, and why it counts as a design value."
url: https://atoneplace.net/reaction-time/aashto-perception-reaction/
site: "At One Place"
---

# The 2.5-second road design figure

**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.

## Related pages

- [Driver reaction to an expected event](https://atoneplace.net/reaction-time/driver-reaction-expected/)
- [Driver reaction to a surprise](https://atoneplace.net/reaction-time/driver-reaction-surprise/)
- [A reflex is not a reaction](https://atoneplace.net/reaction-time/reflex-vs-reaction/)
- [Stopping distance at every speed](https://atoneplace.net/reaction-time/stopping-distance/)
- [Measure your own reaction time](https://atoneplace.net/tools/reaction-time-test/)

## Sources

- A Policy on Geometric Design of Highways and Streets (the Green Book) — American Association of State Highway and Transportation Officials (https://store.transportation.org/)