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• Local Congestion
• References & Experts

Evaluating Congestion in Your Region

If you live in one of the seventy-five regions reported by the Institute, you can use its data as a guide to local congestion. Some of the most important of the Institute's various measures of congestion include:

• The travel time index, which measures the amount of time it takes to make a trip during rush hour compared to the amount of time required with no congestion. An index of 1.2 means that a 10-minute trip without congestion takes 12 minutes at rush hour.
• Per capita (or per driver) hours of annual delay. People's time is valuable and time wasted is one of the most important costs of congestion.
• Gallons of fuel wasted. Cars consume far more gas (and emit far more air pollution) in congested traffic.
• The annual cost of congestion, which is the cost of the wasted fuel, based on local prices, plus the cost of wasted time based on local wage rates.

TTI's congestion data are not based on actual measurements of congestion in every city. The Institute calculates the travel time index and other congestion costs using formulae that assume that freeways, arterials, and other roads have certain flow capacities. When reported uses approaches or exceeds those capacities, the Institute's formula projects that traffic will slow and travelers will be delayed.

This system isn't perfect. Freeways, for example, are not all built alike. The newest roads, such as the Los Angeles Century Freeway, which opened in 1993, can have much higher flow capacities than older roads, such as Connecticut's Wilbur Cross Parkway, which opened in the 1930s. TTI's formulae do not take these differences into account.

This means that TTI's congestion measures are more reliable as a time series for any given urban area than as a comparison across urban areas. While the publicity accompanying each annual update to the mobility report usually focuses on the rankings of urban areas, this ranking is not very reliable.

In the report for 2000, Los Angeles has a travel time index of 1.9, which is significantly higher than San Francisco's index of 1.59. Los Angeles' freeways are so much more heavily used than freeways in most other regions that it is probably reasonable to conclude that Los Angeles has the nation's most congested roads. The next ten urban areas, however, all have indices between 1.40 and 1.47. There is no reason to think that the ranking among these areas is particularly accurate or even that San Francisco's score of 1.59 proves that its congestion is worse than, say, Seattle's (1.45) or New York's (1.41).

TTI's data are more useful for comparing the changes in congestion over time. In most regions, the road network that existed in 1982 (the first year reported by TTI) is still pretty much in place today, so the problem with differences in capacities among roads is less important.

This also means that the best way to compare regions is compare the change in congestion over time. For example, Los Angeles not only has the highest travel time index, it has the greatest percentage increase in this index since 1982.

This measure is also superior because increases in congestion are more stressful than congestion itself. If congestion were constant, people would adjust their travel habits, job locations, or things to compensate. But if congestion is continually increasing, people have to continually adjust or lose more of their time each year.

If you live in a region that isn't included in TTI's annual report, you can still get a rough idea of local congestion using the same federal highway data that form the basis of TTI's reports. These data are published in tables HM-71 and HM-72 of Highway Statistics, an annual report published by the Federal Highway Administration. The federal government, in turn, relies on state transportation departments for raw data, so you may be able to get even more information from the states.

Table HM-71 classifies roads as interstates, freeways, other major arterials, minor arterials, collectors, and local. The table gives the number of miles and miles driven on each type of road. Table HM-72 provides population, land area, and freeway lane miles for each urban area. Unfortunately, the tables do not give the lane miles of other major arterials (most of which are probably four or more lanes wide), but you can probably get these data (which are in the TTI reports for the 75 urban areas reported by TTI) from the state.

Once you get these data, you can divide the miles of travel on each type of road by the number of lane miles of that road type. As shown in the table below, the most heavily used freeways are in Los Angeles, where each lane mile of freeway supports more than 23,000 vehicle miles of travel a day. The most heavily used arterials are in Washington, DC, where each lane mile supports 8,324 miles of travel a day.

These numbers are considerably higher than the average of the nation's twenty or forty largest urban areas and more than twice as high as the twelve least congested areas in TTI's survey. These twelve areas, including Oklahoma City, Spokane, and Anchorage, all have travel time indices less than 1.1.

Daily Vehicle Miles of Travel Per Lane Mile
                                           Other Major
                               Freeways     Arterials
Most heavily used               23,425        8,324
 Region                       Los Angeles   Washington
Los Angeles (TTI=1.9)           23,425        6,621
San Francisco (TTI=1.59)        20,548        7,047
TTI=1.40 to 1.49 (9 areas)      16,987        7,378
TTI=1.30 to 1.39 (10 areas)     17,043        6,249
TTI=1.20 to 1.29 (21 areas)     14,750        6,386
TTI=1.10 to 1.19 (20 areas)     12,294        6,013
TTI=1.00 to 1.09 (12 areas)     10,360        4,941

The table suggests that urban areas with around 17,000 miles of driving per freeway lane mile and 7,000 miles of driving per arterial lane mile are very congested, while 10,000 miles of driving per freeway lane mile and 5,000 miles of driving per arterial lane mile produces very little congestion. The table also suggests that the differences in traffic levels are found mainly on freeways, as the arterials in most regions have about the same amount of traffic.

It is also interesting to note that freeways can produce far more than twice as many miles of travel per lane mile than other arterials. Yet freeways tend to cost only about twice as much per lane mile as arterials, most of the difference being due to the cost of over- and underpasses. Freeways may be the best investment most regions can make in transportation improvements.

The 2001 Nationwide Personal Transportation Survey suggests that an average of 1.6 people occupy private passenger vehicles. So the above numbers can be multiplied by 1.6 to get daily passenger miles of travel per lane mile.

Passenger miles of travel per dollar of investment might be a worthy criterion for comparing highway and transit projects. But it can be misleading because it doesn't necessarily measure real improvements in transportation productivity.

A better measure is the cost per hour of reduced delay. Most regional transportation planning agencies use computer models that allow them to calculate the effects of various road and transit projects on the total daily or annual hours of delay experienced by local travelers. Proposed highway projects in the San Francisco Bay Area are expected to cost anywhere from $5 to $313 per hour of reduced delay. Bus transit projects were expected to cost an average of $11 per hour saved, while rail transit projects cost an average $52 per hour saved.