Damien Sullivan (mindstalk) wrote,
Damien Sullivan
mindstalk

induced demand and transit

So, there's induced demand, the observation since the 1930s that widening a congested road to fix congestion never works. It does do something: it increases bandwidth, allowing more drivers to crawl through congestion. But it doesn't increase speed. Because when demand is high, congestion is the only 'price' that discourages drivers from using the road. Lower the price, and more of them show up until it's high again.

People who learn of this sometimes ask how transit factors in, and I've always said, pessimistically, that it's the same: expanding transit allows more people to bypass congestion -- which is good! -- but won't solve the congestion. Any drivers who take the train will be replaced by more drivers. So the only way to reduce congestion is with actual congestion charges, charging people money to use the road and charging more until you have the levels you want.

But, despite my enthusiasms, I'm an American with American experiences, and I now think I was wrong. At least some of the time. You can watch this Not Just Bikes video or read Wikipedia about the Downs-Thomson paradox or read on:

Imagine that we start with congested roads, and add a subway system that is on average *faster* door to door than driving. As the video says, most people aren't mode bigots (I am! I hate being in cars) and will do whatever is faster (or some mix of faster and cheaper). So if driving is 40 minutes and transit is 25 minutes, lots of people will take the subway. And a good subway system doesn't slow down when heavily used, so it'll keep being 25 minutes even if unpleasantly crowded. Drivers leave the road... and *don't* get replaced, until congestion comes down so much that driving takes less than 25 minutes.

That's the "paradox": "the equilibrium speed of car traffic on a road network is determined by the average door-to-door speed of equivalent journeys taken by public transport." Or by other alternatives, such as bicycling in Amsterdam. If biking were faster than driving, and safe, would you drive most of the time? Probably not. Certainly the Dutch don't. The video guy has said elsewhere that Amsterdam traffic is fairly decent, and this would be why: driving is competing with good bicycling and transit systems, and there's no point to driving to the point that it takes more time than those.

So if you provide good, high-capacity, alternatives to drive, you can reduce congestion!

Now, is this practical? Downs says it's valid in


"regions in which the vast majority of peak-hour commuting is done on rapid transit systems with separate rights of way. Central London is an example, since in 2001 around 85 percent of all morning peak-period commuters into that area used public transport (including 77 percent on separate rights of way) and only 11 percent used private cars. When peak-hour travel equilibrium has been reached between the subway system and the major commuting roads, then the travel time required for any given trip is roughly equal on both modes."


Even for this American, it's hard to imagine such a fast and high capacity transit network. (I was not paying attention to what rush hour driving is like in Osaka.) And it's not like NYC or pre-congestion charge London traffic are/were good, though since NYC transit speeds aren't that high, that might not be a refutation. And of course there are complications: taxis, trucks, and through traffic might contribute traffic that isn't divertible by the alternatives. Conversely, high parking or gas costs could drive people to transit even when slower: what we'd really expect is that people would take the mode that has the lowest cost to them, where the travel cost is a function of time, money, aggravation (overcrowding, weather exposure, being stuck behind a wheel), need to carry things, and maybe more. And as an application of the Law of One Price, people should shuffle between the various modes until the modes have the same average cost.

And this is the problem being an American: we've gone in for cars so much, and scanted the alternative so much, that even with multi-hour rush hours, driving is typically still the fastest and lowest 'cost' mode of getting to work, or anything else. Especially when buses or bicycles get stuck in, or endangered by, cars -- no matter how bad things get, the car is still least bad. This is what killed off streetcars. Subways and bus lanes bypass traffic, as do Dutch (bike paths) or Japanese (wide sidewalks) bicycles.

But as a practical thing... taking a lane of road for bus lanes might actually improve congestion for the cars as well as the buses, by diverting drivers into the buses. If you run enough buses to absorb demand, then the driving speed should come to match the bus rapid transit speed. See the comment count unavailable DW comments at https://mindstalk.dreamwidth.org/581035.html#comments
Tags: cities, public transit
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