Human Transit, a very technical and theory focused transit blog by Australia based Jarrett Walker, this weekend posted and article about the unique benefits that Seattle enjoys when it comes to transit. He uses Seattle as an example of how chokepoints, in Seattle’s case geographically created chokepoints, creates very valuable opportunities for transit to becomes more competitive with cars, compared to cities that have few chokepoints. Jarrett writes:
If you want a real focus for sustainable transport improvements, however, look for chokepoints.
A chokepoint is anywhere in the transport network where many different trip paths have to go through the same point to get past a geographic barrier. Bridges and tunnels are chokepoints. So are mountain passes. Wherever a steep hill is right next to a body of water, the little ledge in between them is a chokepoint, as it often only has room for one road, or a road plus a single track of rail.
No North American city has more chokepoints than Seattle. The city itself consists of three peninsulas with narrow water barriers between them. Further barriers are created by steep hills in most parts of the city. Nowhere in Seattle can you travel in a straight line for more than a few miles without going into the water or over a cliff.
Seattle’s geographical isolation from its suburbs, of course, means it is also surrounded by chokepoints. There are only two bridges across Lake Washington to the east, and to take your car across Puget Sound on the west you have to use a car ferry, which means your trip will be no faster or more frequent than that of a transit passenger.
Transit planning is frustrating in such a place, but road planning is even more so. Ultimately, Seattle’s chokepoints have the effect of reducing much of the complex problem of mode share to a critical decision about a strategic spot. If you give transit an advantage through a chokepoint, you’ve given it a big advantage over a large area.
The lesson of Seattle is that successful transit infrastructure responds to demand, and what drives transit demand is high overall travel demand plus serious barriers to driving. In Seattle, a lot of the barriers to driving take the form of hassle and delay, due to limited capacity through chokepoints.
In other words chokepoints act as an equalizers between transit and cars. Give priority to transit at the chokepoint and you improve its relative attractiveness over cars to a large area. The westbound 3+ HOV lane on SR-520 is a great example.
The relative attractiveness between cars and transit is most commonly model using a logit model. As the attractiveness of transit and cars begin to equalize something interesting happens. Each time transit becomes more attractive, through a reduced travel time of 2 minutes for example, the marginal mode share increases even more than it did for the last 2 minutes of travel time savings. Simply put a travel time savings of 2 minutes is more important when the travel time of car and transit are already close or getting closer. If a car takes 30 minutes but transit takes 50 minutes a 2 minute improvement doesn’t mean much. But if a car takes 30 minutes and transit takes 34 minutes a 2 minute improvement does much more to gain new riders.
This is why Chokepoints like SR-520 eastbound, where transit can travel as fast or faster than cars, especially during peak periods, are places where transit is so competitive with cars. And as Jarrett points out chokepoints are where this confluence of events naturally occur. Read his full post here. There are some good things in it!
UPDATE 1:15 – Jarrett has posted a follow-up piece here.