Perception-Reaction Time

Reaction Time

Robert T. Lynch, PE, Principal Collision Reconstruction Engineer

A colleague recently sent me a link to an online game to test one’s reaction time (see link below). The game simulates a (right-hand drive) vehicle traveling down a straight, hilly roadway with trees, signs, and deer to the sides of the roadway. When the player sees a red stop sign with a hand on it, they are to click the mouse or press any key on the keyboard to stop the simulation. Reaction time, the time between when the stop sign is presented on the screen and when the mouse or keyboard is clicked/pressed, is then compared to how 2,000 people above the age of 18 performed in this same simulation.

The first time I tried the game, my reaction time correlated to someone in their 60’s. However, the game invites you to try again, which I did, and my reaction time improved dramatically, down to what was considered to be consistent with my own age. My best reaction time was 372 milliseconds (0.372 seconds), which was the reaction time of a 28-year-old, according to the data gathered by the game.

While it was a “cool” game to play to see if I could beat my previous time, it does not reflect real-world scenarios where a driver is faced with a sudden emergency, such as a dark-clothed pedestrian standing in the roadway at night. For this game, the player knows what to look for and what to do once that stimulus is presented, simulating a simple reaction.

I recall the “falling ruler experiment” in my high school physics class, which calculates a similar simple reaction time. Drivers who are faced with a hazard often times are not anticipating a hazard to be present at all, and reaction time is prolonged. The unexpecting driver requires time to process the information, decide what action to take, and then act on that decision in an attempt to avoid a collision. The time it takes a driver to perceive a hazard and then apply an evasive maneuver, such as steering or braking, is known as perception-reaction time.

Perception-reaction time for drivers has been well documented within the literature. On average, perception-reaction time for a driver faced with an emergency situation during the day is around 1.5 seconds, whereas perception-reaction time increases to approximately 2.0 seconds for nighttime situations.

Robert T. Lynch, PE, Principal Collision Reconstruction Engineer with DJS Associates, Inc., can be reached via email at or via phone at 215-659-2010.


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