Cheat Day on Your 'Road Diet'

Today I find myself responding to an RFP that will remove a 'road-diet' like cross-section, removing bike lanes, two travel lanes, and a median turn lane, replacing it with four travel lanes, all in the name of road capacity (bike facilities potentially provided off the roadway). I thought the generic part of my write-up would make a pretty useful blog post...

The existing road condition, particularly the section already repaved, is wide enough for four lanes of traffic. In past decades, such space was typically utilised for four lanes of traffic as proposed in this instance, and that would typically result in a high speed road that is generally unpleasant to walk or cycle along, contains many conflicts with side roads, and experiences high collision frequency and severity.

​Many cities today are converting such four lanes streets by way of a ‘road-diet’ to three lane streets with space for cycling. It has been found in studies that due to turning movements from the left lane of a four lane road blocking through traffic, that traffic capacity often doesn’t suffer greatly when reduced to just two travel lanes and a centre turn lane. Furthermore, as the capacity of the signalised intersections at either end does not change the overall throughput of the corridor does not change. More traffic may arrive at the intersection more quickly, but ultimately, the intersections will just see larger queues forming. More drivers may be attracted to the route due to a perceived increase in capacity upon seeing the widening, those queues will just increase even further.

Reverting from the current configuration to a four lane section has the potential to increase vehicle speeds, reduce safety and may ultimately have little effect on throughput along this corridor. The above table taken from TRB Circular E-C019 provides one example of the observed change in traffic performance. In summary, the typically expected benefits of widening are not realized to any great extent.