Running Late? Tailgating at a Traffic Light Will Not Get You Through Faster

It’s important to understand that we move slowly but accelerate quickly.

Credit: quinntheislander

Credit: quinntheislander

Spacing cars only a few inches apart at traffic signals is commonplace. Intuitively, it makes sense – being closer to the traffic lights and packing closely will help get past the lights quicker, once green. A new study debunks this widely held notion.

Investigating this tailgating practice, researchers from Virginia Tech find that tight bumper-to-bumper packing will not increase the driver’s chances of getting through stoppages. Not just that, the odds of rear-end collisions are greater.

Jonathan Boreyko, assistant professor in the Department of Biomedical Engineering and Mechanics, described his frustrating experience of being close to a traffic light one day and yet taking nearly 3 minutes to get past the intersection, as motivation for the study.

This problem, Boreyko and team conceptualised in terms of phase transitions studied in thermodynamics. Basically time and energy are lost in transitioning from this jammed ‘solid’ phase at a traffic light back to the ‘liquid’ phase, before the cars can be moving again.

Published in the New Journal of Physics on November 22, 2017, the study features a series of experiments that involved systematically varying the spacing between 10 identical cars lined up at a traffic signal. A drone camera flying overhead recorded the dynamics of the moving cars. For any given trial, the drivers maintained an initial gap ranging between 1.25 and 50 feet for subsequent cars, but cruised comfortably once the lights turned green.

“What we were looking for was how much time it would take for all ten cars to pass through the lights once it turns green. And if this depended on the initial spacing,” said Boreyko.

Contrary to conventional driver instinct, the team found no benefit in getting closer to the car in front when the traffic came to a halt. “No matter how far apart we spaced the cars, anywhere from one foot to 25 feet, there was absolutely no difference in how long it took for all ten cars to pass through the lights,” said Boreyko.

How come? The trade-off to dense car packing and being close to the lights is that now, one must wait to regain comfortable spacing between bumpers, before accelerating again.

Berthold Horn, an expert in understanding traffic flow and developing road safety solutions at the Massachusetts Institute of Technology, Boston, told The Wire, “Having actual experiments to back up the claim that spacing-out does not speed things up is very convincing.”

The recommendation suggested for efficient traffic flow is simple. Maintain the same spacing as you normally would while driving, as long as gridlock is not an issue. This way you won’t lose time and can gas right away, without having to offset the ‘lag effect’.

However, putting this into practice will most likely be fraught with real challenges, particularly in crowded urban centres. “Maintaining a spacing of 25 feet to 30 feet, that is a two car length, may not be practical in a city because it means fewer cars can fit into the city grid,” deliberated Horn. This would essentially translate to lengthy queues and opportunities for the disobliging to cram within the gaps.

But if you are a pedestrian queued up, say, in a winding airport line, the same rules don’t apply to you. “We found that the closer you cram people together, the faster the line can empty. As it turns out, we move slowly but accelerate so quickly, minimising the lag observed in the case of cars,” explained Boreyko.

Priyanka Runwal is an ecologist and researcher affiliated with the National Centre for Biological Sciences, Bengaluru. She is interested in savanna and grassland ecosystems.