Lower Speed Limits = More Road Congestion

Lower speed limits add to road congestion in several ways, but it’s worth noting that the specific impact will depend on a variety of factors, including the design of the road, the volume of traffic, the nature of the surrounding environment, and the reason for the reduced limit. Here are some of the ways lower speed limits might contribute to congestion:

  1. Capacity Utilization: Road capacity is a function of speed and spacing. If vehicles are traveling slower but are spaced further apart (for safety reasons, for example), the effective capacity of the road might not change much. However, if slower speeds don’t lead to increased spacing, then the road’s capacity could be reduced, leading to congestion.
  2. Variability in Speed: Not all drivers obey speed limits. If a new, lower speed limit is imposed, some drivers will adhere to it while others may exceed it. This creates speed variability, which can result in braking, lane changing, and other maneuvers that disrupt the flow of traffic and can contribute to congestion.
  3. Traffic Calming: Lower speed limits are sometimes implemented alongside traffic calming measures, such as speed bumps, chicanes, or narrowed lanes. While these measures can be effective in slowing traffic and improving pedestrian safety, they can also impede the flow of traffic and contribute to congestion.
  4. Traffic Signal Coordination: In urban settings, traffic signals are often coordinated for certain speeds. If speed limits are reduced without adjusting signal timings, it can disrupt the flow of traffic, causing more vehicles to stop and start, which adds to congestion.
  5. Increased Travel Time: If all vehicles obey a lower speed limit, it will naturally take longer for them to traverse a given stretch of road. The longer vehicles are on a roadway, the greater the likelihood of congestion, especially during peak traffic hours.
  6. Bunching: In scenarios like bus routes, lower speed limits can lead to “bunching,” where vehicles or buses that are supposed to be spaced out end up traveling closely together due to delays at earlier stops. This can create bursts of congestion.
  7. Driver Frustration: Lower speed limits can frustrate some drivers, leading them to make aggressive maneuvers, like frequent lane changes, in an attempt to bypass slower vehicles. Such behavior can disrupt steady traffic flow.

However, it’s essential to note that lower speed limits can also have many positive effects, especially in terms of safety. They can reduce the severity of accidents, improve pedestrian safety, and lead to better environmental outcomes due to reduced emissions at lower speeds. The challenge is to balance these benefits with potential downsides in terms of traffic flow and congestion.