This is an old version of this post, which I've retained to save its comments. See the updated version here.
Now and then, someone mentions that a particular transit project did not reduce traffic congestion, as though that was evidence of failure. Years ago, politicians and transit agencies would sometimes say that a transit project would reduce congestion, though most are now smart enough not to make that claim.
To my knowledge, and correct me if I'm wrong, no transit project or service has ever been the clear direct cause of a substantial drop in traffic congestion. So claiming that a project you favor will reduce congestion is unwise; the data just don't support that claim.
To my knowledge, and again correct me if I'm wrong, there are exactly three ways for a city to reduce its traffic congestion measurably, quickly, and in a lasting way. (Widening roads is not one of these ways, because its benefit to traffic congestion is temporary unless new development in the road's catchment is completely and permanently banned.)
- Economic collapse. Traffic congestion tends to drop during economic slowdowns, because fewer people have jobs to commute to, or money to spend on discretionary travel. A complete economic collapse, which causes people to move away from a city in droves, is always a lasting fix for congestion problems!
- Reduction of road capacity. Ever since the demise of San Francisco's Embarcadero Freeway, it's been pretty clear that if you reduce road capacity for private vehicles, traffic will drop in response. Destroying the Embarcadero Freeway didn't reduce congestion on the parallel surface streets, but it didn't increase it much either. If you reduce road capacity, the remaining capacity is still congested, but this can still be called a reduction in congestion -- especially if you use standard highway metrics like "lane miles of congested roadway."
- Correct pricing of road space. Congestion is the result of underpricing. If you give away 500 free concert tickets to the first 500 people in line, you'll get 500 people standing in line, some of them overnight. These people are paying time to save money. Current prevailing road pricing policy requires all motorists to act like these frugal concertgoers. Motorists are required to pay for road use in time, rather than in money, even though some would rather do the opposite and our cities would be safer and more efficient if they could. Current road pricing policy requires motorists to save money, a renewable resource, by expending time, the least renewable resource of all.
So if transit isn't a cause of reduced congestion, what is its role? Do transit advocates offer nothing in response to congestion problems that have many voters upset? In fact, transit's role is essential, but its effect is indirect.
- Transit raises the level of economic activity and prosperity at a fixed level of congestion. Congestion appears to reach equilibrium at a level that is maddeningly high but that can't be called "total gridlock." At that level, people just stop trying to travel. If your city is car-dependent, that limit becomes the cap on the economic activity -- and thus the prosperity -- of your city. To the extent that your city is dependent on transit, supported by walking and cycling, economic activity and prosperity can continue to grow while congestion remains constant.
- Transit enables people who can't drive to participate in economic life. This includes the disabled and seniors of course, but also the poor. During the US welfare reform debate in 1994-96, government began raising pressure on welfare recipients to seek and accept any employment opportunity. For the very poor living in car-dependent cities, the lack of commuting options became a profound barrier to these job placements. This is really an element of the previous point, since all employment, even of the poor, contributes to prosperity. But this has independent force for government because unemployed people consume more government services than employed people do. This benefit of transit should always be described in terms of economic efficiency, as I've done here, rather than appealing to pity or to alleged "economic rights," as social-service language often implicitly does. The appeal of the social service argument is just too narrow, especially in the US.
- Transit-dependent cities are generally more sustainable than car-dependent cities. They cover less land and tend to have fewer emissions both per capita and per distance travelled. The walking that they require is also better for public health, which produces further indirect economic benefits in reduced healthcare costs.
- Intense transit service is essential for congestion pricing. Congestion pricing appears to be the only effective and durable tool for ensuring free-flowing roads while maintaining or growing prosperity. Congestion pricing always causes mode shift toward public transit, so quality public transit, with surplus capacity, must be there for a pricing plan to be credible.
- Surface exclusive transit lanes (for buses, rail, and arguably two-wheelers and taxis) improve the performance of emergency services. This argument should be much more prominent, because even the most ardent car-lover will understand it. Few things are more distressing than to see an emergency vehicle stuck in traffic, sirens blaring. When confronted with this, all motorists do their best to help. But if the entire width of a street or highway is reserved for cars (moving or parked), and is therefore capable of being congested, it can be impossible to get out of the way of an emergency vehicle even if every motorist present has the best of intentions. Emergency response should be one of the strongest and most obvious cases for surface transit lanes. Motorists understand the need to drop to a low speed in school zones, to protect the life of every single child. Why do we not accept come degree of delay to save a child who may be dying somewhere else, because the ambulance is stuck in traffic?
As far as possible, please present your comments as proposed amendments or additions to what I've written here. I would like to polish my own view on this fundamental question, with the benefit of your thoughts.