Improving the Mass Ave / Memorial Drive Intersection

Summary  With a new intersection layout and revised signal timing at Mass. Ave. and Memorial Drive, through traffic can be reduced to a single lane per direction, creating space for new bike lanes, a bus lane, and protecting bikes from a heavy right turn flow.  With clever signal phasing and coordination with nearby signals, cycle length can be reduced from 110 s to 70 s. Result:  45% less delay for pedestrians, 45% less delay for vehicles, priority for buses, better safety for bikes. Watch the simulation video!

Mass Ave & Memorial Drive in Cambridge needs a bus lane, bike lanes, and protection (in time) between bikes and heavy right turning traffic. More specifically:

  1. Cambridge is required by ordinance to extend its protected bike lanes all the way to Mem Drive.
  2. Cambridge wishes to extend its southbound bus lane all the way to Mem Drive (thus reducing southbound through traffic to a single lane)
  3. The high northbound right turn volume from the Mass Ave bridge onto Memorial Drive creates a constant “right hook” safety hazard with northbound bikes continuing through. Bike safety demands separating these movements in time, which in turn demands a dedicated right turn lane and therefore a single lane for through traffic northbound.
  4. The long signal cycle, together with pushbuttons that pedestrians often neglect to push, results in many pedestrians crossing against the light. The heavy pedestrian and bike demand on the Charles River path demands a good level of service. Because of the long signal cycle (110 s), average delay there is now 44 s, which is too long.
  5. Historic lack of cooperation between the City and DCR means that the signal at Mass Av & Mem Drive (cycle length = 110 s) is not coordinated with the (very) nearby signal at Mass Av & Amherst Street (cycle length = 83 s). Surpisingly, the Mass / Amherst intersection also isn’t coordinated with the nearby signalized crossing at 77 Mass Ave (cycle length = 90 s). That lack of coordination (which requires a common cycle length) leads to spillback queues that create capacity bottlenecks.

With clever intersection design and signal coordination, is it possible to make the needed changes and still provide good service to intersection users? This was the challenge to my students in Fall, 2021 in CIVE 7382, Advanced Traffic Control and Simulation. Hat tip to Ari Ofsevit who suggested this challenge and identifed some inefficiencies in the existing signal timing plan that made it appear feasible.

The long-term effects on traffic volumes of the the work-from-home boom that the Covid pandemic spawned are hard to predict. For this project, students measured traffic volumes in the field on a weekday in November, 2021. Their timing plans and simulations use the traffic volumes found in the busiest 15-minute period in the AM and PM peaks.

This post highlights two of the student’s designs. Edward Orde’s design report develops a solution that works with both AM peak and PM peak volumes. Evan Wotton’s design report focuses on the AM peak, expecting that a similar solution could be used in the PM. Both used the same general strategy of (1) develop a highly efficient signal timing plan for the Mass Ave & Mem Drive and then (2) coordinate that intersection with the other two Mass Ave intersections. Edward’s plan results in a cycle length of only 70 s, far less than the 110 s cycle operating today. Evan’s reduces the cycle length still further, to 65 s, by making use of an innovative treatment for right turns.

WATCH EDWARD’S TRAFFIC SIMULATION VIDEO! See for yourself how much smoother traffic flows, with shorter queues, and better protection for bikes and pedestrians. And WATCH EVAN’S TRAFFIC SIMULATION VIDEOS, a video of existing conditions (AM peak) and a video with his proposed design.

Results (from traffic simulation) are impressive. Here are Edward’s results; Evan’s results are similar. 45% less delay for pedestrians, 45% less delay for vehicles, better priority for buses, better safety for bikes.