A revised and improved version of this post, in response to excellent comments.
Now and then, someone mentions that a particular transit project did not reduce traffic congestion, as though that were evidence of failure. In fact, the relationship between transit and congestion is indirect. (In this post "congestion" means that volume/capacity ratio for motor vehicles on a roadway is high enough to substantially reduce average speeds.) In most cases, it's unwise to claim congestion reduction as a likely result of your proposed transit project.
Road widening, however, is also not a very good way to relieve congestion, except in the short term. In his 1992 book Stuck in Traffic Anthony Downs described the effect of widening an expressway in terms of a "triple convergence":
In response, three types of convergence occur on the improved expressway: (1) many drivers who formerly used alternative routes during peak hours switch to the improved expressway (spatial convergence); (2) many drivers who formerly traveled just before or after the peak hours start traveling during those hours (time convergence); and (3) some commuters who used to take public transportation during peak hours now switch to driving, since it has become faster (modal convergence).
Downs is describing only the immediate effect of the road expansion. Further increases in traffic will come from any new development that is attracted to the road's catchment as a direct consequence of its expansion.
Prof. David Levinson's work in the Minneapolis / St. Paul region suggests that while added capacity generates new vehicle trips, the effect is often not great enough to restore the previous level of congestion. However, your results will obviously vary based on the amount of development that occurs as a result of the new or expanded road. If this development adds enough new vehicle trips to fill the new capacity, traffic congestion can return to near previous levels.
So the only way to make the congestion benefit of new road capacity permanent is to severely restrict development in the catchment area of the road -- an impossible bar in most cases. In fact, parties who will profit from further development in a corridor may be part of the political consensus in support of a road expansion, even as the same expansion is marketed to existing residents as a congestion reducing project.
Otherwise, there appear to be two broadly applicable ways to relieve congestion in a substantial and permanent way.
- Economic collapse. Traffic congestion drops 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!
- Correct pricing of road space. Fundamentally, 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. For example, commenter Brent writes: "Toronto achieved significant downtown employment growth without increasing road capacity after the 1960s, thanks first to increased subway ridership and later due to increased commuter rail ridership. Congestion is still bad on the roads and expressways into downtown, even with transit expansion, but (as you say) the expansion of transit has permitted the downtown to grow beyond what the road network would have supported." A similar pattern can be observed in many similar cities.
- Transit enables people who can't drive to participate in economic life. Groups who don't have the option to drive include many seniors and disabled persons, some youth, and a segment of the poor. Providing mobility to these groups is not merely a social service; it also expands participation in the economy. For example, 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 routinely 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?
In the end, of course, "congestion" is not a good measure of the outcomes of transit. In fact, the very notion of congestion presumes a motorist's view of the world. I agree with commenter Rodrigo Quijada when he writes:
What we'd like to do in a city is to reduce TRAVEL TIMES. Reducing congestion is a way to do that, but in no way the only one. Over the decades, in places where car transportation has become dominant, people have got used to see travel times and congestion as the same thing, thus orienting their thinking and their solutions to reduce congestion. But this is essentially a confusion.
What Rodrigo is describing is simply improved mobility, as defined here. Still, in real-world transit politics, selling transit projects to current motorists is a necessity, and the current motorist is likely to see her problem as one of congestion. So it's important to be clear on what transit can readily do for her.
- It can provide an alternative to driving which may be faster, more cost effective, and less stressful. This argument can be put quite selfishly: Good transit won't eliminate congestion in your city, but it can eliminate it from your daily life.
- Transit helps reduce government spending on social services by enabling transit disadvantaged groups to participate in the economy. This obviously has a range of health and wellness benefit apart from its economic role.
- It can increase the level of prosperity at a fixed level of congestion.
- Its exclusive lanes protect emergency vehicles from congestion-related delays, potentially saving lives.