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Do Aggressive Drivers Speed or Does Speed Make Us Aggressive?

It’s generally accepted that “speed kills.” Urban or rural, single vehicle or multi-vehicle, with or without pedestrians: both the risk of a crash and the severity of that crash increases with speed. What we don’t understand is the interplay of speed, aggressiveness, and time pressure. Advanced immersive simulators are making it possible for driving research to safely explore that interplay in an experimentally rigorous fashion. A 2017 simulation-based study from University of Massachusetts (Amherst) delved into this area, with some interesting results.

Based on surveys, most drivers (~70%) classify themselves as either a “speeder” or a “sometime speeder,” with most indicating they speed either because they are late, they are “in an emergency”, or they are “in a hurry”.

Past studies have further found that people are generally bad at estimating how much time they save by speeding up a given amount (usually overestimating how much time is saved by going from a high speed to an even higher one, while underestimating the gains of going from a low speed to a high speed). They also are poor judges of how much they need to speed up to meet a specific arrival deadline.

Driving Research Exploring Multiple Factors Contributing to “Safe Driving”

For their 2017 simulation-based study, UMass-Amherst researchers Cole D. Fitzpatrick, Siby Samuel, and Michael A. Knodler Jr. designed a scenario lasting around 15 minutes and taking place on a rural two-lane highway with a 40mph posted speed limit and several signalized intersections. 

At two of the intersections drivers were told to turn left while faced with an oncoming vehicle (the point being to test the participant’s “gap acceptance,” or how small of a gap they felt was safe for their left turn). Four of the intersections were scripted to remain red until drivers reached the stop line, and then turn green (an opportunity to assess the driver’s “acceleration profile.”) Near the end of the drive, two intersections were scripted to change from green to yellow when drivers were four seconds away, creating a “dilemma zone” situation. Near the halfway point of the drive, a truck pulled out in front of the participants and traveled a consistent 5mph below the speed limit. Ambient traffic throughout the drive was individually scripted so that oncoming traffic was consistent for all participants. 

Progress updates were placed at the 25, 50, and 75% points of the drive. At each progress update, the driver was shown the percentage of drive completed and the percentage time which had elapsed. 

The driver’s speed and acceleration were monitored throughout. The researchers also assessed the driver’s aggressiveness, based on their behavior at the unprotected left turns while facing oncoming vehicles, acceleration after red lights, treatment of the dilemma zones created by a green light turning yellow roughly four car-lengths down the road, and the slow-moving truck, and their reactions to progress updates throughout the drive.

Studies Show: Time Pressures Increase Speeding

Participants were randomly placed into one of three groups. The control group was told to “drive as they normally would” and that they’d receive $30 at the end of the drive. The two experimental groups were told they’d only receive the full $30 if they 1) avoided any crashes or “traffic tickets” and 2) finished the drive in a fixed amount of time (calculated to force them to hurry). The “Hurried” group was given 16 minutes to complete the drive. The “Very Hurried” group was given just 14 minutes. (It bears mentioning that, because of prevailing ethical standards, every participant got $30, regardless of their performance. The “tickets” weren’t even coded into the system.)

The researchers reported that “the results of this study, with respect to speed, were not unexpected. Drivers who were Very Hurried drove faster. … what was more interesting was that these Very Hurried drivers also selected smaller gaps, were more likely to pass a slow-moving vehicle, and were more likely to run a yellow light than drivers in the control group. This is interesting because speed choice is a conscious choice. If a driver knows they are traveling 60 miles and the speed limit of the road is 60 mph, but they need to arrive in 45 min, they can choose a speed of 75 mph. What is less of a conscious choice are the small and instant decisions, such as whether or not to pass, that are made during a drive. This study has shown that time pressures not only increase driver speeds, but also increase aggressive driving maneuvers which could pose safety risks.”

Time Crunches Spur More Aggressive Driving

Interestingly, this increase in aggressiveness and risk taking was the case even though the Very Hurried experimental group happened to be composed of individuals who were inherently less aggressive drivers, as measured by the ADT questionnaire. After being randomly placed into experimental groups, but prior to driving the simulation, every participant’s baseline driving aggressiveness was assessed using the standard Aggressive Driving Tendency (ADT) questionnaire. While both the Control and Hurried groups had aggressiveness scores around 1.7, the Very Hurried group had a baseline aggression of just 1.56. Nonetheless, this group exhibited meaningfully more aggressive driving when they were crunched for time. The authors note that “while previous survey studies have suggested that time pressures would increase aggressive driving, this is the first evidence, using a driving simulator, suggesting that time pressures affect risk-taking behaviors in addition to speed choice.”