Once again, the Washington region has been tagged as the second-worst place in the nation for traffic congestion.

But it might actually be third or fourth, or perhaps even fifth. Such are the difficulties in pinpointing how 3.2 million cars and trucks move around in a metropolitan area that stretches from West Virginia to Anne Arundel County in Maryland.

When the Texas Transportation Institute released its 1999 mobility study last week, careful analysts did not emphasize the rankings, but instead pointed out the growing costs of traffic bottlenecks in the most congested urban areas, as shown by the study.

No question, the Washington area is on that problem list.

"Is it really second? It's probably in a group that's up very high in the rankings," said Tim Lomax, one of the study's authors.

The institute produces several benchmarks to measure traffic congestion.

Its travel-rate index indicates how long it takes to make a trip during peak travel times compared with off-peak times when travel flows smoothly.

The Washington area's score -- 1.41 -- means that it takes 41 percent longer to get somewhere in rush hour than at other times, the fourth-poorest mark among the areas surveyed.

The institute also calculates the delay motorists experience in rush-hour slowdowns. The average Washington area driver wasted a total of 76 hours in 1997 sitting in traffic, the institute said, the second-worst performance using that measure.

Where do the numbers come from?

The starting point is highway department data on daily traffic volumes measured by sensors placed along urban freeways and major roads. Study authors estimate how much of the day's traffic occurs during peak hours, and from that, calculate vehicle speeds and how much motorists must slow down in rush hour.

But even the institute's analysis, the most comprehensive of its kind, depends on assumptions about what's going on, based on limited samples of traffic flow. Day-to-day traffic flows can vary by 5 percent to 7 percent on a given day, Lomax said.

The margin of error on the "time wasted" tallies is about three to four hours per year, plus or minus. If that variance is taken into account, and the calculations are converted into minutes per day, there's not much to choose between the Washington area, ranked No. 2 behind Los Angeles, and the areas that follow it -- Seattle, Atlanta, Boston and Detroit.

Because the study generates an average for the region's congestion, it can't capture the tremendous differences in the delays individual commuters encounter on various routes to and from work on various days, Lomax notes.

The institute's analysis includes data on accidents and other traffic interruptions in each region, but it doesn't reflect the impact of chainreaction slowdowns that plague motorists when an accident clogs a lane, he said.

"We can't show what happens when there's a bottleneck at one intersection that backs up traffic for a mile," Lomax said. "The level of analysis isn't that specific."

From some lucky Washington area motorists, the drive to work and home again may be better than the institute's average. For many who crawl along the Dulles Toll Road, over the Wilson Bridge or past Silver Spring on the Beltway, it often can be much worse.

Here's one way to tell how you're doing. The institute calculates that the Washington region's motorists average 41.4 miles per hour during peak traffic periods on the Capital Beltway and other freeways, and 26.3 mph on main arterial streets.

If you're doing that well, count yourself lucky.

Clogged Arteries

The Washington area's No. 2 ranking among most congested U.S. cities is based on a calculation of the time an average motorist wastes creeping through rush-hour traffic, according to the Texas Transportation Institute. The differences among the most congested cities shrink considerably when the delays are calculated on a daily basis.

Hours Minutes

per per work

Rank Area year day

1 Los Angeles 82 20

2 Washington 76 18

3 Seattle-Everett 69 17

4 Atlanta 68 16

5 Boston 66 16

6 Detroit 62 15

7 San Francisco-Oakland 58 14

8 Houston 58 14

9 Dallas 58 14

10 Miami-Hialeah 57 14

SOURCE: Texas Transportation Institute