http://www.dallasnews.com/latestnew...road.75239.html
Mixmaster traffic not so bad, data say
07/21/2003
By VICTORIA LOE HICKS / The Dallas Morning News
"On average days, there is bumper-to-bumper traffic for more than six hours ... with average speeds of approximately 20 mph." That was the Texas Department of Transportation's 1998 description of congestion near the downtown mixmaster – and the rationale for building a $600 million toll road beside the Trinity River.
But two sets of data compiled since 1998 found that, except for backups caused by accidents, conditions on Interstate 30 and Interstate 35E near the mixmaster aren't that dire.
For one hour a day in the I-30 canyon and one hour on lower Stemmons (I-35E), the average speed on some freeway segments is about 20 mph, according to a 2000 study by the North Central Texas Council of Governments.
During other "peak" travel hours, speeds on most segments are better than 35 mph, the study said. A 2002 study by the Texas Transportation Institute documented average speeds of 36 to 49 mph during peak periods.
"It's hard to believe it comes out to more than 30 mph," said Tim Nesbitt, the Transportation Department's chief planner on the Trinity toll road project. "From personal experience, I know that [traveling through the mixmaster] your foot is on that brake pedal."
The department's 1998 Major Transportation Investment Study of the Trinity corridor, which concluded that only building a new highway could adequately unsnarl the mixmaster, did not present the underlying congestion data. It was based on a Council of Governments' model that predicts traffic based on population and employment patterns.
The model looks at how many people live in the region, where they live, where the jobs are and a host of other variables, then predicts how many vehicles will use a given road during given hours on an average weekday.
The results are validated in several ways, primarily by spot-checking the predictions against actual traffic counts. Over time, the model has proved to be extremely accurate, said Michael Morris, the council's director of transportation.
The council's 2000 study of commuter traffic took a different approach; it was based on observation. Researchers used aerial photography to document the number of vehicles on Dallas' freeways at various times of day over several days, then counted the number of cars per lane per mile.
Using a formula, they calculated the speed at which traffic on each highway segment was able to move. The greater the density, the slower the speed: 20 cars per lane-mile translates to about 55 mph; 80 cars equals about 20 mph.
The council's photos found no densities as high as 80 (corresponding to 20 mph) in either the canyon or mixmaster. In several spots, photos showed peak-period densities of just 45 cars per lane-mile, corresponding to 50 mph.
However, narratives accompanying the charts generally estimated speeds as being somewhat lower than the numbers alone would indicate. Mr. Morris said the difference may lie in the roads' lousy geometrics.
In layman's terms, they're twisty, convoluted and confusing, forcing drivers to slow down regardless of how much or little traffic there is. Those problems will be improved but not eliminated by a reconstruction now on the drawing boards.
Based on the densities and calculated speeds, each highway segment was assigned a letter grade – A, B, C, D, E or F – for each hour from 6:30 to 9 a.m. and from 3:30 to 6:30 p.m. The grading system was developed by the American Association of State Highway and Transportation Officials.
Under their grading curve – which uses 70 mph as the standard for freeways, regardless of the legal speed – a road gets an F whenever traffic can't move faster than 49 mph. At that point, the freeway is officially congested.
That means the canyon is congested whenever traffic must slow to 6 mph below the legal limit of 55 mph. Put another way, the canyon gets an F when traffic is able to move at 89 percent of the posted limit.
On I-35, where the posted speed is 60 mph, a segment is graded F when traffic is able to move at 82 percent of what's legal.
Those are the standards the Transportation Department and Council of Governments are using when they say the freeways around the mixmaster are congested.
City Council member Lois Finkelman, who is on the council's transportation committee and participated in the 1998 investment study, said even public officials may be hazy about what highway planners mean by "congestion."
As a result, she said, "our discussions may be a whole lot less valid than they ought to be."
Ms. Finkelman, who serves on the Council of Governments' Regional Transportation Council, said she's surprised that 70 mph is the standard for all freeways – especially given that no freeway in Dallas has a posted speed of 70 mph.
"We could have some discussion about whether that is an acceptable base rate, or if it should be ratcheted down," she said.
More broadly, she said, it's valid to ask: "What's an acceptable level of congestion during a rush-hour period? Some people would argue that if you can travel at 40 or 45 mph, that's pretty good."
In a perfect world, with unlimited money, Mr. Nesbitt said, highway planners would add enough lanes so that every freeway, during every hour of the day, achieved what he called "ideal, driver-preferred conditions," meaning you never have to hit the brakes.
In the imperfect real world of downtown Dallas, they are willing to accept an F during four hours of the day. The toll road along the Trinity – supplemented by improvements to existing freeways, more mass transit and other strategies – is supposed to make sure that's still the case in 2025.
"We know we can't design a perfect system, but we want to design the most efficient system we can," Mr. Nesbitt said.
Mr. Morris said that even if the Transportation Department could add enough lanes to I-30 and I-35E to handle additional vehicles – and it can't, he said – having a second highway is a better solution.
Having one freeway is like having one hose, he said. If an accident clogs it, you've got no alternative route.
"The Trinity is the second hose," he said.
E-mail
vloe@dallasnews.com