BACKUP IN THE PASS: New Est. Up to 19,000 People at Once in I-10 'Fiasco'

A new estimate from UC Irvine's Institute of Transportation Studies puts the number of people stuck in the 20-mile, 10-hour backup Sunday on westbound Interstate 10 at about 19,000 people at any one time.

The estimate is based on several assumptions, and should be considered a "ballpark" estimate, Prof. Michael G. McNally of UC Irvine's Institute of Transportation Studies said Thursday.

"Yes, at one point in time," McNally said in an email exchange. "If you took an aerial photo of the entire 20 mile stretch, you would see about 19,000 people in 13,600 cars and trucks, given the various assumptions involved.

"Most of these 13,600 cars would each be in this queue for about 2 hours, assuming a congested speed of 10 mph, with another 13,600 cars joining the queue every two hours," McNally said.

Many motorists said they took four to six hours to cover a distance they could normally drive in 45 minutes. Unannounced construction on a pavement rehab project in Banning led to the backups, which stretched 20 miles into Palm Springs, according to the California Highway Patrol.

There are several ways to estimate the number of cars on a freeway, and there are several assumptions necessary, so results can vary and they should be considered nothing more than estimates, McNally said.

"The first approximation is assuming a linear relationship between speed (miles per hour) and density (vehicles per mile) to ease computations," McNally said. "This relationship may be linear at mid-ranges of speed but probably not at low speeds with stop and go traffic.

"Second, we need to specify a mean free speed and a jam density," McNally said. "I assume that the I-10 corridor in that area has a mean free speed of 70 mph and that jam density is about 200 vehicles per mile. Both of these figures may be a bit low for that location.

"Third, using a linear speed-density model with the above two parameters yields a density of 170 vehicles per mile at 10 mph and yields a density of 185 vehicles per mile at 5 mph. These figures are per lane . . .

"Using the 10 mph result of 170 vehicles per mile per lane, and assuming 4 lanes and a 20 mile backup, yields 13,600 vehicles on the I-10 WB in the queue at a single point in time," McNally said.

"If you assume 1.4 persons per car (I'm not sure but it seems reasonable for Sunday night traffic returning from the desert), that's a little over 19,000 people," McNally said. "This does not reflect changing queue length over the 10 plus hours of the event."

McNally, an associate professor of civil and environmental engineering, specializes in transportation systems analysis, travel forecasting models, complex travel behavior, transportation and land use.

McNally's estimate of 13,600 vehicles is more than three times greater than an estimate from Postdoctoral scholar Andre Tok, also of UC Irvine's Institute of Transportation Studies, who studied Banning-Beaumont Patch photos and vehicle distances on Wednesday.

Caltrans has traffic detectors on some of Southern California's busiest freeways, but Tok researched location of the detectors Wednesday and found none on the stretch of Interstate 10 that backed up eastbound from Banning.

"There are no detectors in that area that report traffic data," Tok said. "The furthest east traffic detector station along the I-10 freeway is located in Beaumont, which is still west of Banning."

McNally's and Tok's estimates, and others, will be part of ongoing discussions as motorists who were caught in the Sunday chaos, Caltrans and California Highway Patrol officials, and elected leaders seek answers and accountability for what went wrong.