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"Hmm, the shopping list says that I need some Worcestershire sauce, and some lettuce, and cake mix." But all in all, traffic engineering is unlike going shopping for groceries:

A recent traffic-calming installation on Sparks Street in Cambridge is more like an attempt to prepare a dinner from these ingredients:  "oh no, the Worcestershire sauce makes a poor frosting for the cake, and a poor dressing for the salad. Darn, I can't make anything at all from this!

Sparks Street between Brattle Street and Mt. Auburn Street in Cambridge is one-way, and passes through a residential neighborhood. The City of Cambridge has installed curb extensions (bulbouts) so that parking shifts from one side of the street to the other at the end of each block. The purpose of shifting the lane position is evidently to slow motorists by preventing them from driving straight.  Note the parking lane on the left in the foreground in the photo below, and on the right in the background.

A parallel-parked motorist can more easily get in and out when the driver's seat is on the curb side. But when exiting the parking space, and more importantly, the motorist can more easily see and yield to overtaking traffic if the driver's seat is on the street side -- as is usual on two-way streets.

The City has installed a bike lane on Sparks Street, always on the right side. Two vehicles shown in the photo are continuing straight in the bike lane rather than slowing down and jogging over to the left. More than a dozen vehicles passed me as I took the photos on this page. All of them drove in the bike lane.

Sparks Street at Foster Street, Cambridge, Massachusetts, USA --
Bike lane jogs left, motorists don't.

DSCF0175 Sparks Streetsm.jpg (38663 bytes)

The traffic law requires drivers to merge to the right curb before turning. However, as the photo below shows, a solid white bike lane line extends up to the crosswalk before the intersection, and the bike lane has dashed borders only inside the intersection, directing motorists to turn right from the left lane. The bike lane also directs bicyclists to swerve left. A knowledgeable bicyclist would merge left before the intersection in order to establish right of way and avoid conflict with right-turning motorists. Now, that would slow overtaking motorists. But the premise of traffic calming is to constrict vehicle movement in order to slow traffic -- rather than to allow the normal interaction of vehicles on the road to slow the traffic.

How might motorists be made to jog left? A barrier between the bike lane and the rest of the street would accomplish that, but it would constrain bicyclists to enter the bike lane in the door zone after the intersection, and would interfere with turning movements by large vehicles.

Bicyclist directed to swerve across path of right-turning motor traffic.
Photos taken April 14, 2005

DSCF0180Sparks Stsm.jpg (38541 bytes)

Lessons of this design:

  • Segregating slower bicycle traffic by means of bike lanes removes the natural traffic calming effect of mixed traffic.

  • Chicanes don't work in conjunction with a bike lane, because the added width of a bike lane defeats the chicanes. Motorists will drive in the bike lane to avoid zig-zagging down the street.

  • When the parking repeatedly switches sides on a one-way street, it is not practical for bicyclists to ride on the side opposite the parking to avoid car-door conflicts.

  • It is generally preferable for parking on a one-way street to place the driver on the street side.

  • The City of Cambridge continues to disregard the traffic law which requires drivers to merge right before turning.

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