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How AVs Could Impact Congestion: The Pros and Cons

Published on March 1, 2023

Autonomous vehicles (AVs) look set to become a common sight on roads all over the world in the near future, but there's still much research to be done to understand what impact they could have on our roads. A major point of discussion is whether or not they could improve the state of our increasingly congested cities and motorways. AVs have many pros and cons, but is their overall impact on congestion likely to be positive or negative? Read on to find out.

1. Pro: AVs could stop unnecessary traffic jams

Sometimes traffic jams occur due to accidents or roadworks, but in many cases they happen for seemingly no good reason. This is because even small disruptions to the constant flow of traffic can lead to a chain reaction that dramatically slows or even temporarily stops movement. When a driver taps the brake unnecessarily, they initiate a wave of tapped brakes in the line of traffic behind them. This creates what is known as a "stop-and-go wave". 

AVs are designed to optimise their speed and distance to maintain traffic flow without unnecessary braking, which means they could help to prevent traffic jams in areas susceptible to congestion.

2. Con: AVs could worsen traffic jams on roads with multiple lanes

Some studies into the impact of AVs on congestion are positive, with results demonstrating that congestion could be reduced when even a small number of AVs are in use alongside human traffic. However, a 2021 study from the University of Western Australia found the opposite to be true on double lane roads. 

AVs leave large gaps in between them and the vehicle in front of them specifically to avoid unnecessary braking. The study found that human drivers changed lanes to cut in front of AVs more frequently due to this large gap. Each time they did so, the process triggered a stop-and-go wave and inhibited the smooth flow of traffic.

3. Pro: AVs could 'platoon' to free up space

Platooning is the process of multiple AVs driving in a coordinated way with very small gaps between them. Their ability to communicate with one another allows them to maintain high speeds safely despite these small gaps. 

This means that a group of vehicles could take up less space overall on a road in order to ease congestion and maintain flow. Platooning is likely to be most useful on fast-moving roads, such as motorways. Truck platoons are already being tested throughout Europe using automated driving technologies such as automatic braking.

4. Con: AVs could inhibit traffic flow if they don't work together

When AVs work together they can optimise traffic flow and increase speed of traffic by as much as 35%. However, this process relies upon the vehicles seamlessly communicating and responding to one another. When AVs don't cooperate with one another, they generate stop-and-go waves and queues just like human-driven vehicles. 

Car manufacturers are each developing their own unique AVs and associated software. If manufacturers don't ensure their vehicles communicate and cooperate successfully with those from other brands, AVs could have little impact on congestion. Since manufacturers are competing with one another, they may not be incentivised to work together to ensure AV cooperation.

5. Pro: AVs could reduce rates of accidents

Accidents tend to cause a great deal of congestion as roads are fully or partially closed for hours at a time. The majority of road traffic accidents are caused by human error. Temporary lapses in judgement, failure to notice hazards and reckless driving behaviour can all lead to accidents. 

AVs continually monitor the roadway and automatically respond accordingly to minimise the risk of accidents. In theory, AVs can make roads safer by reducing rates of human error. If there are fewer accidents on the roads thanks to AVs, congestion rates will fall simultaneously. Plus, human life could be better protected. 141 cyclists and 346 pedestrians were killed on UK roads by cars in 2020 – AVs could help to reduce these numbers in the future.

There's much to learn about the impact of AVs

Interactions between vehicles are incredibly complex and vary greatly depending on the unique conditions of each road and journey. For this reason it's impossible to know for sure whether AVs could improve or reduce congestion overall. It's likely that we won't fully understand the impact of AVs until greater numbers of them are on the roads and being operated by average consumers.

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