These roadways have performed well by some traffic standards, but problems remain.

American cities are years away, at best, from adopting congestion pricing programs on downtown streets, like those used in Stockholm and London. But road pricing has been in effect on various metropolitan area highways since the mid-1990s. These take the form of high-occupancy toll lanes, which are usually free to carpools or buses but can be used by single-occupancy vehicles for a fee, or peak-pricing lanes, which carry a greater cost during rush hour to encourage travelers to drive at off-peak times. These projects seem to be gaining popularity; there are about 400 miles of priced highway lanes in operation, with 11 HOT lanes under construction.

The nonpartisan Government Accountability Office recently completed an audit of 14 highway pricing projects (5 HOT lanes, 9 priced lanes) in U.S. metro areas. The resulting report [PDF] found evidence that highway pricing programs can reduce congestion, but it also found some reasons for concern.

These included an inability to isolate the cause of traffic reduction in some places, a decrease in passengers per car in certain corridors (suggesting a heavy amount of single-occupancy car use) and potentially negative impacts on lower-income drivers. In a word, the results found by the GAO are mixed:

The 14 congestion pricing projects that have current and complete evaluations generally show that pricing can help reduce congestion, although other results are mixed, and not all possible relevant impacts have been assessed.

Hot lanes. Travel time and speed have improved on at least part of all 5 HOT lane corridors evaluated in the GAO report. Cars traveling in HOT lanes do move quicker than those in unpriced lanes, in keeping with the aim of such programs: in San Diego, for instance, HOT lane users saved 20 minutes over unpriced lane travelers, by one recent estimate. Still traffic flow improved in many of the unpriced lanes too. In Seattle, rush hour speeds on SR-167 rose as much as 19 percent, the GAO reports.

In some metro areas, however, the root of the traffic gains are less clear. In Miami, for example, drivers saved 14 minutes in HOT lanes and 11 in unpriced lanes over previous travel times. But Miami's HOT program coincided with the construction of entirely new lanes, which may skew the statistics.

While building a lane has short-term traffic benefits, the "fundamental law of road congestion" suggests that new lanes fill up quickly. In other words, the improvements realized in Miami might be the result of the HOT lanes, or they might be a fleeting relief provided by the new lane. If the latter, then one should expect Miami's lanes to crowd once more, requiring higher HOT lane prices to maintain steady flow.

The GAO also found that HOT lanes have increased vehicle throughput in certain corridors.

In Minneapolis, the volume of cars moving through the I-394 program area increased 13 percent on HOT lanes and 5 percent on unpriced lanes. But four of the five HOT projects also showed a decrease in the number of passengers per car in a corridor. That means instead of pooling up to avoid HOT lane charges, many travelers are simply paying more to drive in those lanes alone. HOT lane pricing expects (and even requires) a certain amount of single-occupancy activity, but encouraging more of it seems antithetical to the spirit of such programs.

Peak-pricing. Interestingly, the GAO found that peak-hour pricing programs had no effect at all on travel time or speed. Slight improvements on the Jersey Turnpike, for instance, were attributed mostly to the implementation of electronic tolling. Likewise, peak-pricing lanes didn't increase vehicle throughput, according to the report. 

Of course the primary goal of peak-hour pricing is to encourage people to drive at non-peak times, and in this regard the programs seem to have achieved some modest success. In the New York metro area, for instance, surveys conducted by bridge and tunnel authorities found that 7.4 percent of drivers had changed their travel time to a non-peak hour — usually before the morning rush.

That's enough to cause a potential change in flow. But even this shift isn't clearly attributable to pricing programs, according to the GAO. While some drivers might travel at off hours because they want to pay a lower toll, others might do so simply to avoid traffic. The first reason would substantiate the goal of peak-pricing programs; the second is a natural outcome of tons of traffic.

Transit and Equity. The most discouraging revelation in the GAO report is that neither HOT nor peak-hour pricing programs seem to create a rise in transit use. (The lone exception was Miami, where express bus ridership rose 57 percent between 2008 and 2010 — with two in five of these riders being former single-occupancy drivers.)

In addition, the GAO had concerns about the equity of pricing programs. While drivers of all income levels used lanes in places Orange County, Minneapolis, and Seattle, high-income drivers still used them more. One potential response to the equity problem is to shift some of the money generated by priced lanes to transit programs, such as express bus service. There seems to be at least some evidence that the public approves of such policies, according to the GAO, which cites increased residential support in Seattle for highway tolls tied to transit improvements. 

All in all, the report concludes, metro area transportation authorities need a bit more time and stronger evaluation methods to understand all the benefits and drawbacks that priced lanes create:

Pricing has the potential to reduce congestion by influencing drivers to carpool, use transit, or drive at off-peak travel times. Congestion pricing has, where evaluated, helped reduce congestion. However, it is difficult to draw overall conclusions about the effectiveness of pricing because only half the sponsors with projects now open to traffic have evaluated their projects. Other results, where available, are mixed, as project sponsors have used different measures to assess performance and little has been done to compare performance across projects. Where congestion pricing projects have also added lanes, the results of pricing have not been distinguished from the results of adding capacity. Finally, congestion pricing’s impact on traveler behavior and equity has yet to be fully explored.

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