Speed, Tunnel Vision, and Reaction Time

Part 2 of our series on Building Safer Streets

How reducing vehicle speeds saves lives
Visual perception at different speeds, streets.mn

By Ian Thomas

One specific way in which higher speeds impact safety, especially in complex driving environments, is through the effect of speed on a driver’s visual field.

When a vehicle is traveling slowly (say, 15 mph), the driver is able to receive and interpret a lot of visual information from the periphery of his or her field of view.  Therefore, a child stepping off the curb or another car pulling out of a driveway would be picked up very quickly, allowing the driver to prevent a collision in most cases.

However, there is a limit to the rate at which the visual system and brain can process signals.  At higher speeds, the vehicle travels a greater distance down the street in the same amount of time.  Therefore, fewer visual signals from the periphery can be processed, and the child or car will not be seen so quickly.

When this reduction/delay in visual information is combined with the increased “stopping distance” due to the driver’s fixed reaction time and higher speed, there is an increased risk of a collision. And a high-speed collision is much more likely to cause fatalities.

This is less of a concern when driving in the more controlled environment of an interstate highway.   The absence of pedestrians, driveways, cross streets, etc., intentionally simplifies the context.  In other words, the narrower “cone of vision” at higher speeds is less of a problem because there is very little happening in the peripheral field of view. 

Traffic engineers, (and businesses near fast roads) know about your ability to process information at high speeds.  Check out the size of the signs they put up as compared to local streets and how those signs are far in advance of the action they want you to take. They know you can’t process anything smaller, and certainly not quickly enough to change behavior in time.

You can test this phenomenon yourself. Stand at a corner, with or without a crosswalk, where auto speeds are slow, then stand at a corner where auto speeds are fast.  On the calm street drivers are much more likely to slow down, make eye contact, and try to determine your intentions.  On a fast street, they are much less likely to slow or stop, either because they don’t see you, or slowing or coming to a stop feels like a sudden action to them.

In more complex environments, that limitation on our ability to “read” our environment and react appropriately becomes a deadly problem. Attempts to control human behavior through education, enforcement, or exhortation will ultimately fail because humans simply are not perfectible in this way. The tragic consequences of normal human inattention or mistakes means we must design streets to slow traffic to sensible speeds when people outside of cars are present.

Read the previous installment of this series on our blog.