Thursday, April 22, 2010

Chicanes on Columbia Street, Cambridge

Submitted By Mahmoud El Hazek

Columbia Street is a two-way local roadway that intersects Massachusetts Avenue in the city of Cambridge, Massachusetts. The roadway is a north-south route that cuts through a residential neighborhood that is primarily occupied with houses, and a public playground and park for the younger residents and teenagers in the area. The roadway contains one 11 ft travel lane in each direction, with parking space on the east side of the corridor until Washington Street intersection. The city of Cambridge has implemented two chicanes along Columbia Street between Washington Street and Worcester Street, which are spaced at approximately 225 ft apart. By definition chicanes are curb extensions that alternate from one side of the street to other forming s-shaped curves. The chicane effect on Columbia St. is achieved by shifting the parking space between the east and west side so that drivers need to turn between the shifting parked vehicles.

One key feature of the chicane treatments along Columbia Street is the signage. Before every chicane, a sign is placed to alert drivers that the turns exist so that hopefully they would slow their speed if necessary. The signs are placed in both directions so that vehicles travelling northbound and southbound are exposed to the sign before they hit the chicane.

The chicane on Columbia St. is designed and intended to act as a traffic calming measure that will reduce vehicular speeds, or ensure that the speeds are not exceeding the 30mph limit. More specifically, drivers are intended to reducetheir speed in order to safely turn in both directions while remaining in their correct travel lane. Drivers that are driving too fast may face difficulty turning and can either collide with the parked vehicles on Columbia Street or not be able to recover from a turn and may engage in a head on collision with opposing vehicles that may ‘surprise’ them. The fact that these chicanes were implemented may suggest that residents/city were frightened from high speeds in this highly residential corridor. Other emphasis on this treatment could be:

  • To reduce speeds close to the public playground as children are usually walking to and from their homes to access this facility
  • To ensure safety of pedestrians walking near the roadway
  • To reduce speeds in order to allow for drivers to stop or react incase of a parked driver opening their door suddenly – the narrow roads does not provide any ‘dooring’ space between travelling and parked vehicles

The chicane treatment may be beneficial, however is not needed along Columbia Street. Firstly it is important to note that along the first intersection when travelling right from Mass Ave there is a flashing red light, which forces drivers to come to a complete stop and may proceed only if there are no conflicting traffic or pedestrians. This light is in fact not needed as at that same intersection there is a stop sign, which essentially will force vehicles to completely stop as well. With this light, vehicular speeds are calmed and there isn’t enough time and distance for vehicles to pick up speed to the next intersection, as a flashing yellow light is placed at the right before the first chicane just after Washington Street and Columbia Street. These collective measures enforce vehicles to enter the chicane turns at relatively low speeds (approximately witnessed at 20mph) in the first place. Although it is arguable that the chicane ensures that the speeds are kept low, I believe with the excessive signage and flashing lights for awareness already existing a cheaper solution could have been implemented, like speed humps.

The design objective is also not met as drivers witnessed on Columbia Street tend to encroach on the opposing lane while driving along the chicane, so that they maintain a straight-line path across the centerline without needing to slow down. This is possible as while out on the field it was witnessed that two cars crossing the chicane in the opposing directions at the same time was very rare (I witnessed this event twice or three times only), and so drivers had the freedom to encroach without any fear of collisions for most of the time.

This design could be possibly improved by making reverse curves sharper and longer. It is clear that the chicane curves on Columbia Street are wide enough to defeat its purpose of slowing down the traffic. Another alternative design or adjustment to the chicane is the implementation of a raised median that may prevent the current problem of drivers encroaching on the centerline. However to implement this impeller concept, parking may be eliminated to provide space for the median. Furthermore, as stated earlier the calming design used could be exchanged for a cheaper one that would maintain lower speeds in the residential community, especially since chicanes are expensive due to the high costs of curb realignment and right of way. Two speed humps spaced 200 feet apart will reduce speeds along the corridor in this case as well as two chicanes spaced 200 feet apart and which have low compliance from drivers. Initially the corridor has a stop sign placed at the preceding intersection, flashing yellow and red lights, and raised crosswalks, and narrow lanes that all ensure approach speeds into the chicane are much less than 30mph, which further reinforces the unnecessary need of a chicane treatment in this investigated area.

The traffic is already calm throughout, I believe that the calming treatments are over implemented on that stretch of the corridor. A video attached additionally shows that vehicles are of relatively low speeds before approaching the chicane already which raises questions of the necessity of an expensive measure over other treatments that may maintain the ALREADY calm traffic in the neighborhood.

1 comment:

  1. If you observed traffic speeds being calm, I wonder why you don't credit the chicanes for that. There obviously must have been a speed problem before, which prompted the need for traffic calming, and now it's gone.

    Red flashing lights (same as stop sign) are doubtless effective. To lump them together with a flashing yellow and call them effective together is a failure of analysis.

    Nice movie clip illustrating the weakness of a chicane on a 2-lane road without a raised median. Cars just cut the corner. However, the chicane still keeps the road from being the straightaway it used to be, which helps calm traffic.

    Also, parking friction acts as a speed reducer. Before, one side had all the friction and the other side had none; combined with a straightaway, that's a recipe for high speed. Now both sides get some parking friction.