In the May 28 blog summary of the GPA’s transportation committee’s first meeting with SFMTA, traffic engineer Damon Curtis had promised to look into two things:
–creating simultaneous protected lefts northbound and southbound on Diamond
–switching the new protected northbound left on Diamond to be southbound in the evening to accommodate rush hour traffic
Neither option will be possible. The rationales given by SFMTA are summarized below (and Curtis’s full email is attached here).
One important term to note: LOS means “Level of Service,” a state standard that basically measures how many cars can be pushed through an intersection in a given time. It is graded from A to F. Its applicability is changing in a way that I didn’t totally understand, sorry to say (hw).
On creating simultaneous protected lefts northbound and southbound on Diamond:
Curtis modeled simultaneous protected northbound and southbound left turns with both a 90-second and a 120-second signal cycle. For the former, the intersection level of service [LOS] degrades to F and for the latter it degrades to E. Therefore both scenarios trigger a significant impact that would require additional, extensive environment review.
A protected NBLT does not degrade LOS to an E or an F because northbound traffic can still proceed straight through the intersection at the same time. (See more detail in the longer answer below.)
On switching the new protected northbound left on Diamond to be southbound in the evening to accommodate rush hour traffic:
Technology exists to allow for different left-turn phasing at different times of day to meet changing demand, but based on the traffic counts from the Glen Park EIR Traffic Impact Study which SFMTA used as the basis for its recent left-turn analysis, northbound left turn volumes far exceed southbound left turn volumes in both the AM and PM peak hours (229 versus 137 in the AM, and 210 versus 117 in the PM).
More from Curtis:
“In traffic signal timing we base the amount of green time for a given movement on the volume of cars and the movement that requires the most amount of green time is defined as the critical movement. If we base the timing on percentages, the intersection would not operate efficiently and inevitably one or more approaches would begin to back up.
“For the NBLT and SBLT at Diamond/Bosworth, the left-turns as a percentage of all traffic on their approach are about equal, but we know that beneath those percentages are real numbers and that the NBLT has a demand of 229 cars and the SBLT demand is 137 cars. That means the NBLT has 67% more vehicles than the SBLT. In addition, in the NB direction there are 243 vehicles going straight or turning right, but in the SB direction that total is only 165, i.e., the NB demand is 47% greater.
“And since cars going straight and cars turning right must share a single lane (the case for both NB and SB), it only takes having that first vehicle in the queue wanting to go straight to hold up all of the potential right-turners and therefore the expectation is that the majority of the straight thru and right-turn movements will occur only when the light is green.
“Taking all these factors together, we can begin to understand why the intersection Level Of Service breaks down when we introduce a protected SBLT phase – it’s primarily because during a protected SBLT phase, all of the NB straight thru and right turn traffic must be held back, then during the subsequent green phase we are unable to provide enough time to serve all of those vehicles, not to mention serving the demand for EB and WB, and still keep the overall signal cycle length below 120 seconds.”
GPA: What about congestion backflow into GP Village?
Curtis: “Regarding favoring southbound [protected left] over northbound because the effects of congestion are greater in the residential/commercial area north of the intersection than they are coming off the freeway to the south, I will start by saying say that when it comes to the signal timing at Diamond/Bosworth there is no magic bullet and there will be some trade-offs.
“I discussed this very point with Ricardo [Olea, City traffic engineer] and we both agree that when taking a more macro view of traffic operations at and around Diamond/Bosworth, there is little question that causing back-ups on NB Diamond (which having a protected SBLT would do) will negatively impact traffic on Monterey as far back as the freeway on/off-ramps at Monterey/Circular, and that would have a far greater impact on a much larger number of people than the alternative.”
[Note from Heather] If you’ve read this far, you’re a true geek and might want to check out this presentation from Ricardo Olea titled “Signal Timing and Pedestrians.”