Regarding autonomous vehicles (AV), traffic engineers and transportation planners often focus differently on AV startups, original equipment manufacturers, and private companies.
Public sector representatives, traffic engineers, and transportation planners desire an efficient, safe, and equitable transportation system. In contrast, the goals of the private sector tend to focus on capturing market share and generating profit, which often incentivizes companies to prioritize customer experience over efficiencies in the traffic system. While private sector companies may provide safe and efficient transportation methods, the good of the community is not always the private sector’s primary directive.
Adopting AVs will almost certainly generate more vehicle miles traveled (VMT) than the alternative case, where AVs are not widely utilized. Given an increase in VMT, AVs only improve traffic if their adoption increases road capacity.
AVs’ Impact on Road Capacity
In a highway setting, one of the most significant factors of road capacity is the vehicle headway or the space between vehicles. AVs can drive closer and maintain a safe distance because of the vehicles’ fast perception, reaction time, and more precise maneuvering.
The difference between actual safety and the perception of safety can impact the consumer experience — akin to a rollercoaster feeling dangerous while still being safe. Just because an AV can follow another AV 10 feet apart while going 80 miles an hour doesn’t mean the rider will be comfortable. This driving behavior may prevent consumers from purchasing AVs or using a service that drives in that fashion. Therefore, the private sector would have no incentive to program self-driving systems to reduce headway between vehicles.
Achieving the benefit of reduced headways will require a higher adoption rate of AVs, where the vehicles can either form platoons or have a dedicated lane. This behavior is not practical in a mixed-traffic environment composed chiefly of human drivers. The AV industry will have to overcome the safety perception and the mixed traffic flow before this benefit can be realized. While it is possible that infrastructure such as dedicated lanes could support those behaviors, that would require a public sector investment with no guarantee that the AVs would fully take advantage of it.
Features like adaptive cruise control and the smooth driving of an AV can improve stop-and-go traffic and reduce or eliminate phantom traffic jams in traffic models. However, this effect does not appear to play out on the roads. It is unlikely that a vehicle manufacturer would optimize its system to improve traffic flow of its own volition. AVs are designed with the individual driver experience in mind. Manufacturers want the vehicle to drive safely, smoothly, and efficiently. Any positive or negative impacts on the traffic stream are likely unintentional or a secondary concern.
Another capacity factor that must be considered is speed harmonization. Roadways perform better with some natural variation in speed. It is essential to create space between vehicles so there is room to merge and change lanes. If all AVs are programmed to travel at the same speed, it could reduce this natural variation, making it harder for vehicles to find gaps and openings. This could create moving roadblocks, resulting in backups, congestion, and reduced access. Additionally, if AVs are not programmed to be able to speed — as that would be illegal — then slowing down is the only way to create a gap in traffic. This would reduce the overall speed on the road and, in turn, the roadway's capacity.
The most significant impact on road capacity on surface streets is from bottlenecks at intersections. One major factor affecting the capacity of an intersection is the acceleration of the vehicle from a stop. The faster vehicles get to speed from a stop; the more vehicles will be able to make it through a green light. A relatively minor change in the vehicle operating characteristics can enormously impact traffic performance. In one interchange project, the only difference between a mile-long queue and a functioning interchange was the acceleration rate of the trucks that used that interchange. If AVs react quicker to a green light, start in unison, and accelerate quickly, this could significantly improve capacity. If, however, slower acceleration is preferred — because it provides a smoother ride — that could significantly reduce the capacity. Again, overall traffic system performance is likely not a top priority, as the private sector is programming self-driving behavior.
Making each user safer does improve the safety of the entire traffic system. In this regard, the public and private sector values are well-aligned. Increasing safety by reducing crashes is one of the most promising aspects of AVs. The crash reduction also benefits traffic flow by eliminating congestion due to lane closures and traffic diversions. This ultimately improves travel time reliability.
Safety is one of the paramount benefits of AVs; however, the impact on the more considerable traffic and transportation system is yet to be determined. These challenges will require innovative transportation solutions and interdisciplinary collaboration. Be it the development of new policy, infrastructure investment, or market signals and monetary incentives; no two locations will require the exact solutions. Context-specific elements, such as existing infrastructure, vehicle type, population density, and alternative transportation options, will influence decisions. Through continued public-private collaboration and thoughtful, forward-looking planning and investment, we can capitalize on the strengths of AVs to improve our transportation systems.