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Andres Power from the SF Planning Department provided some more information on the left turn lanes on Cesar Chavez, and how these were determined.

We were curious as to the size of the left turn pockets, as well as if U-Turns would be allowed. There was some confusion over U-turns being allowed at Folsom, and hence if traffic destined for Harrison or Alabama would end up going down Precita or 26th.

Andres told us that generally where there is a dedicated left turn pocket with a left turn arrow, u-turns are permitted. This will likely be the case at Folsom, although the plan has not been formalized yet. It should be noted that Bryant has a No U-Turn sign currently.

We gathered week-long 24-hour traffic counts at all the intersections along Cesar Chavez Street about three months ago.  This gave us the typical 24-hour distribution of car volumes, including left turns.  At Folsom, during the peak PM hour there were 39 cars turning left onto Folsom in the westbound direction and during the peak AM hour there were 30.  Again, these counts reflect the peak AM and PM hours.  At all other times of the day, volumes were about half.  But, as a matter of course, traffic engineers typically design the roadway for peak functions. In addition, if we assume that all cars that would have otherwise turned left at Alabama and Harrison will now turn left on Folsom (which is a conservative assumption, since many of the cars, as you point out, will make a u-turn at Folsom), the total left turn AM peak into Folsom will be 69 cars per hour and 73 cars per hour for the PM peak.  

You can do the math, but this distributes out to a little more than 1 car per minute turning left from Chavez into Folsom during the peak hours.  This is in contrast, for example, to the 213 cars turning left during the peak PM peak at Mission and the 208 cars turning left at Bryant. [Editors note: the left turn at Bryant is largely used by people getting onto 101 southbound from westbound Cesar Chavez.]

Therefore, with the volumes indicated above, and assuming a 90 second green-red-green phase, the cue, as modelled, during the peak hour would typically be between 3 and 4 cars.   The turning pockets are designed to include a buffer, hence the designed length of five to six cars.

He also clarified some information on the streets that would have left turn pockets: The original plan was to include left turns at Bryant, Folsom, South Van Ness (eastbound only) and Guerrero. Recently, left turn pockets at Mission Street and one at Valencia Street were added.

Labels: Cesar Chavez, Precita Valley Neighbors, traffic calming