How roadway capacity and safety intersect with Vision Zero goals

The goal of Vision Zero initiatives is to have no deaths or serious injuries from traffic collisions. This is an important target for the municipalities in the region that have adopted a Vision Zero policy. One challenge to meeting this marker is to address two dual needs for transportation — capacity and safety.

What is capacity and safety?

In transportation, capacity is the ability of a roadway to accommodate a volume of vehicular traffic. It’s related to traffic flow and throughput; i.e. how many vehicles can travel through a specific point over a specific time. It’s also related to congestion and delay, or how much additional time it would take for a vehicle to travel a certain distance if there is congestion. Safety is related to reducing the likelihood of crashes in the transportation system.

Capacity and safety are not on opposite ends of the same spectrum. Roads can have few crashes and can handle lots of vehicles, or conversely, have low capacity and low safety. For instance a freeway can have proportionally fewer crashes, while a rural two-lane road can have proportionally more crashes.

When transportation engineers and designers for roadway agencies, departments of transportation, and property developers design roadways, there are many variables that are taken into account; and two of these are capacity and safety.

Is capacity prioritized over safety?

There is a portion of Pennsylvania Avenue (Route 4) in Upper Marlboro, Maryland where there are two travel lanes and a shoulder in each direction. During the AM peak travel period (6:00 t0 10:00 AM), people are allowed to use the shoulder as a third travel lane. So, the capacity of the road is increased, which helps reduce traffic jams during rush hour. However, by doing this, it prevents people from stopping in the shoulder if their automobile should break down, and if there was a serious collision, it could also prevent an emergency vehicle from reaching the crash. This is an example where roadway capacity and the reduction of congestion is emphasized.

Capacity is also prioritized when intersections are designed to reduce instances where vehicles may have to slow down or stop, such as intersections with channelized “free-right” turns.

A channelized “free-right” turn is an intersection design where instead of a motorist stopping at the intersection and then turning right onto the cross street, a separate lane is used to connect the two cross streets, creating a triangular-shaped island in the intersection, and allowing the vehicle to turn onto the cross street without having to fully stop.

This design allows motorists to turn at higher speeds, or turn without stopping, and therefore increase the capacity of the intersection. However, this design also means that pedestrians must cross another lane of traffic, one in which motorists are traveling at higher speeds and do not stop. Even with marked crosswalks and signs this can be an example where roadway capacity for motor vehicles is emphasized.

This balance between capacity and safety is one result of the many metrics used by roadway designers and engineers to determine the attributes of a street. A common metric is Level-of-Service, or “LOS,” which is a ratio between the capacity of a roadway and the number of vehicles that can use the roadway before it becomes “congested.”

A road with very little traffic congestion ranks LOS “A” and a road with lots of traffic congestion would rank as LOS “F.” While the LOS ranking is supposed to be an unbiased assessment of capacity, it is difficult to disassociate A-to-F from passing-to-failing, especially when an intersection with an LOS of “F” is commonly called a “failing intersection.” If an intersection is “failing” from a LOS perspective, roadway agencies will often require additional projects to widen the intersection or change its geometry so that it has a higher capacity, i.e., so that more vehicles can pass through it more quickly.

Additionally, there are other factors considered when roads are designed. It is not just balancing safety and congestion. For instance roadway engineers also take into account vehicle-to-vehicle crashes, how to reduce the total number of overall conflict points between all modes at an intersection, as well as considering how the design of one roadway segment or intersection will affect or be affected by other segments in the network.

It is also important to note that traffic safety extends beyond the realm of engineering. For example, one of the Six E’s of Prince George’s County’s Vision Zero program is education; ensuring that everyone using the roadway - motorists, bicyclists, transit users, and pedestrians - understand how to safely use the streets together.

What does Vision Zero do about this?

Since the goal of Vision Zero is to eliminate traffic fatalities and serious injuries, collisions that result in death and serious injury are more important to prevent than reducing congestion. Vision Zero further identifies the design of transportation infrastructure as an approach to achieving safer streets, which represents some of the greatest opportunities to make advances in safety.

Candidly, Vision Zero may be difficult to achieve. And while most jurisdictions - Washington DC, Montgomery County, Prince George’s County - do well to clearly articulate an overarching Vision Zero Policy, it’s equally important for the Departments of Transportation, Police Departments, and other agencies to use Vision Zero to adjust the detailed practices, like re-evaluating roadway capacity, in order to prioritize safety.

Adopting a Vision Zero policy is clear guidance from elected officials to roadway engineers to take safety into account, even if some roads and intersections would see more congestion. Following this policy still maintains that roadway engineers are able to use appropriate engineering judgment in order to make sure new transportation infrastructure fits within the context of its surroundings while eliminating traffic fatalities and serious injuries.

Prince George’s County, where I live, is off to a good start implementing its Vision Zero policy. Using a data-driven approach, the County has put together an informative crash data map and a two-year action plan for moving forward. Hopefully, the County and state can use these policies to design and build roads that advance safety for everyone.

Thumbnail: Image by the author.