When it comes to autonomous vehicles (AV), traffic engineers and transportation planners often have a different focus than AV startups, original equipment manufacturers and other private companies.
As public sector representatives, traffic engineers and transportation planners desire to have 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 methods of transportation, the good of the community is not always the private sector’s primary directive.
The adoption of 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, the only way that AVs improve traffic is if their adoption results in an increase in road capacity.
AVs’ Impact on Road Capacity
In a highway setting, one of the biggest factors of road capacity is the vehicle headway, or the space between vehicles. AVs can drive closer and still maintain a safe distance because of the vehicles’ fast perception and 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 has the ability to follow another AV at 10 feet apart while going 80 miles an hour doesn’t mean the rider will be comfortable in that situation. This type of 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 the self-driving systems to reduce headway between vehicles.
To achieve 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. In a mixed-traffic environment composed mostly of human drivers, this behavior is not practical. The safety perception and the mixed traffic flow are two factors the AV industry will have to overcome before this traffic flow 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 be playing 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 impacts, positive or negative, to the traffic stream are most 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 important 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, which would result 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 capacity of a roadway.
The biggest impact to 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 have enormous impacts on 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, can start in unison and accelerate quickly, this could greatly 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 self-driving behavior is being programmed by the private sector.
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. Crash reduction also provides a traffic flow benefit by eliminating a lot of congestion that occurs because of 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 larger traffic and transportation system is yet to be determined. These challenges will require innovative transportation solutions and interdisciplinary collaboration. Be it development of new policy, infrastructure investment or market signals and monetary incentives, no two locations will require the same solutions. Decisions will be influenced by context-specific elements, such as existing infrastructure, vehicle type, population density, alternative transportation options and many more. 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.
AVs that can operate on the entire roadway system may still be a long way off, but many are already being deployed today. Learn more about the specific areas and specialized use cases where the vehicles are being utilized.