The Reaction Time Paradox

It’s 6:00 am and already the road is clogged with the cars of commuters. Traffic ebbs and flows between short bursts of acceleration, deceleration, and quick stops. Drivers weave in and out of lanes, each vying to gain an extra foot closer to their destination.

Inside the cars radios, podcasts, streaming music, text messages, navigation prompts, and phone calls compete for the driver’s attention, while outside lane-splitting motorcycles and electronic billboards draw eyes and more than one moment of panicked reaction.

With so much going on, so many stimuli fighting for our attention, and the need to quickly react on the road, it’s no wonder that patients who have suffered injuries need to be evaluated by occupational therapists before getting back behind the wheel of a car.

Luckily, there are tests OTs can use to measure important functions and reactions without actually putting the patient on the road and endangering anyone. Tests that can be administered in a safe and controlled environment, yet will still provide the data OTs need to accurately evaluate whether or not a patient should be allowed to drive a car.

These tests can be administered through a driving simulator, and should measure a couple of different parameters:

  • Simple Reaction Time
  • Complex Reaction Time

Simple Reaction Time Test

The data most commonly referred to comes courtesy of simple brake reaction time tests. These tests are called “simple” because the only task the participant must perform is to watch for and respond to a stimulus by removing his or her foot from the gas pedal and then, as quickly as they can, press the brake pedal. All of his or her attention is focused on quickly responding to an expected stimulus.

Simple reaction time tests have been the gold standard for decades. However, when the RT-2S Brake Reaction Timer was tested on 400 drivers, authors of the study noted, “In reality, attempting to determine the absolute brake reaction time for an individual is probably not possible.” The results from this study have been summarized in Figure 1.

Average Brake Reaction Time by Age Group

This is a problem. Occupational therapists need to be able to make recommendations based on data that gives some clue to how a driver will do in the real world, on a real road, with real distractions demanding a piece of attention.

Complex Reaction Times

As anyone who’s ever driven a car can attest, driving is not a straightforward task. Drivers must be aware of any number of stimuli: the speed of their car, the speed of the cars around them, brake lights, reverse lights, lane changes, signs, pedestrians, street lights, and on, and on.

This leads into the concept of complex reaction times. Most, if not all, situations while driving require drivers to divide their attention between a multitude of things all happening at once. Which means simple reaction time values do not translate well to real-world applications.

Perception vs Reaction Time

Figure 2 above shows data for drivers who encounter an unexpected situation as they crest a hill. If a pass/fail system was created based on the simple reaction times from Figure 1, every driver would fail. They were performing an actual complex driving task and not focused on a simple braking task – which is why their reaction times were longer.

In situations where a driver has to track numerous potential threats, reaction times become even longer. This inconsistency between simple and complex reaction times leads to a Reaction Time Paradox.

The Reaction Time Paradox

The fact that simple and complex reaction times do not compare to each other isn’t bad unless the mistake is made to directly compare the two. Some reasons for this discrepancy:

  1. Complex reaction times have stimuli that are often hard to detect at first. All reaction times begin with a stimulus being presented to the driver. In the cases of simple reaction times, the stimulus is easy to see and the driver is expecting it. When on an actual roadway environment, it’s more difficult for a driver to pick up stimuli that are subtle. This results in longer reaction times.
  2. To make roadway environments realistic and unpredictable, critical driving situations often have other elements that make detecting the stimulus for a reaction times more difficult to spot, thus elevating reaction times. Even in situations where the driver is boxed in and their only response is to brake to a situation, the reaction times can be elevated simply by the rich graphical environment and focusing on a different element of the drive besides what is ahead of them on the road.
  3. Drivers are unpredictable which leads to unexpected behaviors, no matter how well critical situations are designed. For example, a driver might not feel safe in a particular situation and may already have their foot on the brake when a critical situation presents itself; resulting in an abnormally fast reaction time. In the same situation, a different driver may decide that steering is the correct response resulting in possibly no pedal activity at all.

And these are just a few reasons why reaction time measurements differ between the simple and complex. So how can we relate a driver’s simple reaction time values to meaningful performance in the simulation of a complex environment?

  1. If you have a participant who will be driving the simulator multiple times, you can compare their earlier performance to later performance. As a participant progresses through their rehabilitation process, you should see improvements in their performance across similar tasks.
  2. Use their reaction time measure along with other measures such as Time-To-Collision (TTC) to assess their performance during critical events.

There are driving schools who claim that if a driver had an additional second, 90% of all accidents could be avoided or greatly reduced. Therefore, if a driver’s TTC is greater than one second, they have a huge advantage when trying to avoid dangerous situations. With data from a driving simulator, the TTC measure can be compared to a driver’s simple reaction time to bridge the gap between complex and simple reaction times.

If a driver has a TTC greater than one second, then they have more than likely given themselves enough time to be able to react and handle something else occurring during this critical situation. If, on the other hand, their TTC values are less than their simple reaction time, they are dangerously close to being involved in an accident and would not have the capacity to handle another immediate critical situation.

For most situations the following table illustrates what the TTC values mean.

TTC Collision chart

Simple reaction time tests are easy to administer and have been the industry standard for years, but they aren’t an accurate reflection of real-world driving capabilities. By using both simple and complex reaction time data, OTs can gain a greater insight into how a driver will perform while out on the road and decide whether or not a patient is ready to get back behind the wheel.