Martin’s post about signal priority on the First Hill Streetcar brought to mind an exchange I had with SDOT staff about signal priority on the RapidRide D line, which will replace Metro’s current Route 15. In particular, I raised an issue that an STB commenter had mentioned to me, namely that the inbound left turn from Elliott Ave W onto W Mercer Pl (where the bus turns from Interbay to make the deviation through Lower Queen Anne) is, under its current configuration, likely to cause unacceptable delays:
Regarding the D Line (Downtown-Uptown-Ballard), there is a signal at Elliott & W Mercer Pl that, from 2 PM to 8 PM, presents a very long (3-4 minutes, I believe) red signal to traffic turning from Elliott to Mercer. This will have a dramatic effect on the speed and reliability of the inbound D Line unless it is fixed. What kind of signal priority will the D Line receive at this light? If it’s the usual ~15 second hold-or-advance-green it will be completely inadequate at this light.
Here is the response I received from Enrique Garcia, Signal Timing Engineer at SDOT:
The reason for the wait time on the southbound left turn movement is that, in the PM peak commute hours, there is a high influx of northbound traffic. Our goal is to provide progression on the main arterial for this large traffic volume. This may result in side street and left turn movements having to wait a bit more. The idea is that once you are on a main arterial with good progression, the stopping should be minimal. I monitored this location and although the wait time can be a few minutes for the southbound left turn, the signal clears out the queue once the movement is serviced.
We are currently working on the re-timing of signals for the installation of the D Line Metro project. The specifics have not yet been finalized, so at this time we cannot say what kind of signal priority will be implemented at this location. We will, however, be taking an in-depth look at the timing and adjusting it to accommodate all modes and directions of transportation as efficiently as possible.
This answer, while totally reasonable as a discussion of engineering trade-offs, does not address* the policy question of whether good signal progression for arterial car traffic is more important than ensuring speed and reliability on a bus route that is being held forth by Metro and the City as a flagship BRT route. Ultimately, it doesn’t matter whether your transit runs on steel or rubber tires, what matters is whether it’s frequent, reliable, and a speedy way to get in to, out of, and (most importantly) between dense urban areas. SDOT aren’t to blame for RapidRide’s 15-minute off-peak headways, but they have have a lot of say in how fast and reliable the C, D and E lines will be, and Elliott & Mercer is where we shall find out exactly how much the City of Seattle is going to Walk, Bike and Ride the talk.
* And I’m not faulting Garcia — I wouldn’t expect an SDOT staff member to address a policy question such as I’m doing in this post.