A Primer on Angled Parking Math for San Francisco’s Traffic Engineers.

After two meetings with SFMTA traffic engineers, local neighbors and bike crash victims to discuss the problem of Muni track caused bike wrecks on 17th Street a third meeting was convened in Supervisor Scott Wiener’s Office. This was to be the one where the SFMTA team lays out their proposed solution to the problem. The date was October 6, 2016.

So a bunch of us took the afternoon off and went downtown to participate. What was the big plan, the result of four months of meetings and work?

A half baked verbal proposal to take all the cars off both sides of 17th street for exactly one block (Church to Sanchez) to run semi-protected bike paths along the curbs over the broken to hell concrete parking strip.

No ideas whatsoever to deal with the Muni tracks in the intersections or the remaining blocks of tracks to Castro Street were put forward. Nothing about the angled parking conversion proposal for Sanchez Street. Not a clue about traffic calming at the intersections or the mid-blocks.

Not a single document or drawing. They were literally empty handed. No less than four men paid well over a hundred thousand dollars a head after getting four months lead time. (Years depending on how you count.)

That’s right. Not one drawing or serious solution proposal. So we had a lot of time to kill and I started asking detailed questions. I was basically following up on each topic discussed to date to see if these guys had gotten anywhere on anything. I was probing.

The guys from “Livable Streets” were notably challenged when it came to basic math.  One would think these would be the people to have an intelligent conversation with about the impact of angled parking on lane width and traffic dynamics. wiggle-reverse-angle

The thing to know about angled parking is that it eats up more road space than 90 degree perpendicular parking does. This holds true until you angle the vehicle so acutely that you eat up all your curb space thus killing your capacity to store a lot of cars.

That is why perpendicular parking is so popular in the Castro. It stores the most cars in the least space while allowing for 12 foot travel lanes for traffic (fat & fast). The downside is that it is dangerous in several specific ways. This is why many communities have moved on to reverse angle parking.

Sanchez Street between 17th Street and Market is a scary stretch for local bicyclists because cars back out blindly from these perpendicular spots into City Bicycle Route 47 which is  shared by vehicular traffic. The obvious solution is to institute back-in angled parking. We did a whole post on this with graphics and everything.

Sanchez Street front first perpendicular parking backing out onto City Bike Route 47.

But Sanchez Street is only so wide. The current arraignment optimizes both parking and traffic lane width. Angling the parking impacts both of these numbers negatively.

Therefor, when planning changes you have to consider the width of the traffic lane first because the Fire Department has big trucks and cannot be blocked. But if you angle the cars so far that the road width is increased or kept constant then you are removing a lot of parking and may increase congestion and traffic by causing drivers to cruise locally for parking.

In other words, to plan for reverse angled parking on Sanchez Street we have to sit down and do the math to pick the angle that works.

Reverse angle parking is safer. You can see the cars and traffic in front of you when it is time to pull out!

So imagine my surprise when I raised the topic at the meeting in Supervisor Wiener’s Office and the Livable Streets guys literally shouted me down in unison saying “Angled parking widens the street.” Both men were emphatic on the point, treating it as a platitude, a conversation killer.

(It turned out they have an agenda: they don’t want to even consider the angled parking thing at this time.)

I was trying to be non-confrontational so in frustration I turned to the one senior engineer at the table and asked if back-in perpendicular parking is an option. He said it is but compliance would be problematic. (The angle reinforces the back-in concept.) Mike is experienced and tactful.  He even flew to Copenhagen with Jane Kim last year! Needless to say, he wasn’t inclined to correct his colleagues’ arithmetic.

Kim Sue Sallaberry etc
Mike Sallaberry in orange in Copenhagen with Supervisor Jane Kim and SFMTA Board of Directors Member Gwyneth Bordon.

We left it at that but the conversation left a bad taste in my mouth. These four traffic engineers laughing at me made me think I’d better check my own math, maybe I missed something. They were so confident.

It turns out they were so focused on the politics they got the math wrong.

I could try to explain using trigonometry. Essentially for any angle (x) you choose from 1 to 89 the distance from the curb to the rear bumper is easily calculated.

(Sin (x))(length of car) + (sin (90-x))(width of car) = distance from curb to rear bumper.

But that’s pretty dry stuff. Let’s drop the trig lesson while we’re still awake (and don’t even ask about curb overhang).

Or one could just google diagrams of angled parking lots and look at the measurements.

Just one example of many showing the relationships between angle chosen and distance from curb. This engineer used a chart to lay out the options.

But I thought, after doing both of these things that it would be more fun to just measure my car at various angles against the curb and see what that shows.

Perpendicular 90 degree angle. Rear bumper is 148 inches from the curb.
Park at 50 degrees and the rear bumper is 169 inches from the curb.
Park at close to 40 degrees and the rear bumper is 161 inches from the curb.
Park at close to 30 degrees and the rear bumper is 137 inches from the curb. (But you are taking up 16 1/2 feet of curb space.)
Parallel parking takes 83 inches to the outside of front tire and 12 feet 4 inches of curb space.

Considering the importance of competent traffic safety engineering in San Francisco where we are hemorrhaging blood on our streets and killing someone every two weeks perhaps we should consider some minimal requirements to work at SFMTA.

Right now it is the “go along to get along” standard where no one blows any whistles and everyone gets paid.

Or perhaps we should investigate private contracting alternatives to these highly politicized civil service jobs. Other communities have improved their transportation system massively by introducing the element of competition in design and planning. Perhaps San Francisco should go this way.

Consider a system in which the knowledge base of street capacity, measurements, and other design and operating details were available to private contractors. Imagine a system where The City sets transparent honest goals and hires private companies to plan and modify the transportation grid in a certain way by a certain time.

What would it be like to live in a city that set concrete goals, had them planned professionally and implemented on a schedule by private contractors operating within a budget?

We wouldn’t know here in SF. Our guys can’t plan a parking lot, much less build one so we just muck along and try not to think about how much money and time we are wasting on this current system. We treat people getting killed trying to use the streets as an inevitability that should be accepted.

We live in a place where people are being killed so often by bad traffic engineering that the local media doesn’t even report their names or personal stories anymore. This is what happens when you politicize your traffic department and fill it with guys who don’t know basic math.

Car versus pedestrian on Market Street by Safeway. November 5th, 2016.

This is San Francisco where our city traffic engineers refuse to discuss local safety upgrades. The few they agree to get deferred forever. I think we deserve better. If SFMTA can’t put a safe parking option alongside  City Bike Route 47 then maybe we should outsource the job to a private contractor and get it done now. And perhaps other  bike path work should be outsourced as well.

Just in: San Francisco 2012-1015 Collisions Report — November 3, 2016.


  1. Really great write up of the meeting and the issues around parking space street use. I can only hope this reaches the right ears. I’m with you on the need for serious improvements on 17th, not just half-baked and lazy “fixes.”


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