Skip to Content
Streetsblog San Francisco home
Streetsblog San Francisco home
Log In

More Cars = Less Congestion? Supes Grill CPMC’s Perplexing Traffic Analysis

How can more cars relieve congestion at an intersection?

Members of the Board of Supervisors pursued that question this week during two hearings on California Pacific Medical Center's plan to build the massive new Cathedral Hill medical center at Geary Boulevard and Van Ness Avenue. The board postponed a vote on Tuesday to approve the project's environmental impact report, which was approved by the Planning Commission in April despite the 1,200 parking spaces it would include.

The thousands of car trips those parking spaces would induce have troubled residents and livable streets advocates. Scrutinizing the traffic impacts predicted in the EIR, supervisors grilled Planning Department staff over the perplexing finding that congestion at two intersections would actually be reduced, despite additional vehicle traffic.

Viktoriya Wise of the Planning Department made numerous attempts to explain the perplexing "peak-hour factor" used in Level of Service, the automobile-centric traffic formula which planners must use to analyze traffic impacts. But Supervisor David Chiu wasn't satisfied, asserting that the peak-hour factor wasn't used in other cities, according to experts he'd spoken to.

However, supervisors stopped pressing the question after an explanation from Greg Riessen, also of the Planning Department. "This is such an arcane little element of traffic engineering, that, in my six-year career, has never got anywhere near this much attention, so it's very exciting to be here and explain this to you," said Riessen, eliciting chuckles from the board and the public. Assuring members that the often counter-intuitive formula is used around the country and on other projects in San Francisco, he explained (here's an edited summary):

The peak hour factor is trying to quantify the peak 15 minutes of congestion within the peak hour. If you put more cars through the less-congested approaches at an intersection, the overall weighted average of delay could go down sometimes. Our standard practice is to increase the peak-hour factor to represent the spreading out of traffic as your fluctuations decrease over time, but sometimes that'll result in results that don't necessarily jive with your intuition. It's kind of more a question of, why does Level of Service do that? And it's frankly one of the reasons why we need to abolish Level of Service.

Clear enough? Seemingly satisfied with Riessen's explanation, the supervisors moved on.

The project's development agreement includes a fee on parking: $0.75 for every car during peak hours, and $0.50 during off-peak hours. That money would go to the SFMTA for local pedestrian and transit improvements, including the Van Ness and Geary BRT projects. CPMC will also pay the city $400,000 to study bicycle improvements on connecting corridors. The parking fee, however, is set to end after 10 years.

The EIR is set to return to the board for approval on July 31.

Stay in touch

Sign up for our free newsletter