More comfortable car trips mean more congestion

February 3, 2015

Self-driving cars are coming. Drivers are already willing to forfeit the wheel, as we’ve previously blogged, and a self-driving Cadillac appears to be less than two years away. A new study investigates whether two key benefits that self-driving cars promise are likely, based on data from light rail transit and high-speed rail: less traffic, and freeing up time that would otherwise be spent driving.

A recent study uses data from other modes of transportation to assess how self-driving cars will impact traffic. Image from nanny snowflake.

A recent study uses data from other modes of transportation to assess how self-driving cars will impact traffic. Image from nanny snowflake.

Two of the top benefits of self-driving cars are increased road capacity (reducing road congestion), and extra in-car time for the car’s occupants (what the study authors call “a range of leisurely and economically-productive activities that are either not possible at all while one is driving, or are not as productive while driving because the driver must continuously devote a share of his/her cognitive resources to driving-related tasks.”)

Rail is a prime example of a form of transit that lets passengers devote time to “leisurely or productive activities,” since passengers have no driving responsibilities. Comfortable passengers, suggests the study, are better equipped to work productively or otherwise enjoy their commutes; a self-driving car that provides a smooth ride, similar to a high speed rail ride, would ostensibly create greater comfort for its passengers.

Hailing from the Department of Geography at SUNY New Paltz, and the Centre for Transport Studies of the Department of Civil and Environmental Engineering at Imperial College London in the United Kingdom, the study “Autonomous Cars: The Tension Between Occupant Experience and Intersection Capacity” highlights the trade-offs between ride quality in self-driving cars, and traffic.

Part of the reason train travel time can be leisurely or productive is because the ride is generally smooth. Yet if a driverless car were to drive as consciously — “speeding up and slowing down more smoothly for the rider’s sake,” as CityLab explains — traffic would be delayed as a result, a sacrifice for the car occupants’ comfort.

One of the study’s authors Scott Le Vine explains to CityLab: “Acceleration has big impacts on congestion at intersections because it describes how quickly a vehicle begins to move… Think about being stuck behind an 18-wheeler when the light turns green. It accelerates very slowly, which means that you’re delayed much more than if you were behind a car that accelerated quickly.”

The study’s researchers simulated traffic at a four-way city intersection, with 75% of cars driven by real people, and the remaining 25% of cars driven autonomously. In certain scenarios, the autonomous cars started and stopped similarly to light rail trains, which is to say, somewhat jerkily; in others, they started and stopped similarly to high-speed rail — very smoothly. (The study also analyzed other options, such as longer yellow lights, for improving smoothness of the ride and reducing speed.)

The study did not include pedestrians, whose presence means even more starting and stopping. Yet it still found that driverless cars which mimicked the most comfortable train rides negatively impacted road congestion. When praising the benefits of self-driving cars, the study suggests, consider that some of these benefits — reduced traffic and occupant comfort — may be at odds: The more comfortable the ride, the more congested the road.

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Category: Transportation

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