MoPac's 'managed lanes' might also cost you a pound of flesh
Special lanes in Dallas have sharply increased accidents, but engineers say MoPac plan would address flaws.
April 23, 2007
By Ben Wear
If the state adds two "managed lanes" to MoPac Boulevard north of Town Lake as proposed this month, the increasingly clogged freeway would have a third more capacity and a high-speed refuge for buses, emergency vehicles and motorists willing to pay tolls.
However, given design compromises forced by lack of space, research indicates that the highway would also have more accidents and serious injuries. What is not clear at this point, because little or no study has been done of the particular design contemplated for MoPac (Loop 1), is just how many more accidents.
"If you did not have right-of-way constraints, would you design it differently?" asked Bob Daigh, Austin district engineer for the Texas Department of Transportation. "You bet you would. But that's not the project we have to design. We have to live within those constraints."
The agency hopes to begin construction in late 2008 and have the lanes open two years after that.
The constraints exist only in the southern five miles of the project, between Town Lake and RM 2222, where Union Pacific railroad lines on the inside and a combination of enormous power poles and resolute homeowners on the outside mean the expansion must occur within the Transportation Department's existing right of way.
North of RM 2222, there is plenty of room for expansion, and the state will be able to lay down full 12-foot-wide lanes and spacious 10-foot-wide shoulders.
For that southern section, however, Daigh and his engineers have spent the past year or more trying to figure out how to cram in another lane, plus a buffer of a few feet between the new managed lane on the inside and the three lanes that would remain free. To do that, the design would narrow the regular lanes to 11 feet and have shoulders that typically would be 4 feet wide.
In the worst case — on the northbound lanes of a milelong stretch between West 35th and West 45th streets — the inside shoulder would be a scant 15 inches wide, the managed lane would be just 11 feet wide, and the normal 4-foot buffer separating it from the free lanes would be just 2 feet wide.
For cars using the managed lane in that area, the total side-to-side maneuvering space, including the lane itself, would be just over 14 feet.
The Texas Transportation Institute, in a 2004 study of accidents and designs on Dallas high-occupancy-vehicle lanes where there is typically no inside shoulder, recommended that cars on such segregated, high-speed lanes have 26 feet total side-to-side maneuvering space. That would allow cars to get around a stalled vehicle or one slowing down to move over into the regular lanes.
The "absolute minimum cross-section," the report says, should be 18 feet.
"Without an inside shoulder, when you looked at the crash reports (in Dallas), there just wasn't anywhere for someone to avoid the crashes," said Scott Cooner, an associate research engineer in the Texas Transportation Institute's Arlington office.
The study looked at accident rates on Interstate 35 and Interstate 635 before and after HOV lanes were added. After the HOV lanes opened in 1996 and 1997, injury accidents per million miles traveled over the next four years increased 41 percent on I-35 and 56 percent on I-635.
But as with so many questions about highway safety, any comparison to what might happen with MoPac's managed lanes is necessarily inexact. The state Transportation Department, to some degree because of experience with the Dallas HOV lanes, would build the Austin lanes differently.
On those two Dallas highways, the only separation between the HOV lane and the regular lanes is a double stripe painted on the pavement. Signs tell drivers not to cross those solid double stripes, that movement from the HOV lane to the inside regular lane is supposed to occur only every mile or so, when there is an access point indicated by a dashed line.
On MoPac, the managed lane would be segregated from the regular lanes by a series of closely spaced, flexible plastic pylons. At entry or exit points — and there would be only five, aside from the southern and northern ends — there would be a gap in the pylons of about 1,200 feet, about a quarter-mile, where people could make the lane change.
The reality in Dallas, according to the 2004 report, is that many people have ignored those signs, weaving in and out of the managed lane in efforts to gain advantage or (in the case of people driving alone who are illegally in the HOV lane) to avoid being caught and ticketed. Most of the accidents, Cooner said, occurred because of that rampant lane changing.
The fundamental problem is that cars in the HOV lanes at rush hour, by and large, are going 30 to 35 miles per hour faster than cars in the regular lanes, Cooner said. That speed differential makes lane changes more problematic than on a normal freeway, where everyone typically is traveling at the same speed.
That same problem would exist with the managed lanes on MoPac.
"If you're going to ask people to pay, they have to be going faster," Cooner said.
In fact, the plan with MoPac is to have "dynamic pricing" to ensure that speeds remain high on the managed lane. The tolls to drive in the lane would be significantly higher at peak traffic hours, set at whatever price it took to discourage enough drivers to keep traffic uncongested.
On State Route 91 in Southern California, for instance, the toll for a 10-mile stretch of managed lanes varies between $1.10 overnight and $9.50 between 4 and 5 p.m. Fridays.
The expectation with MoPac is that the pylons, by limiting lane changes to the designated access points, will produce a far lower accident rate than Dallas has seen. In fact, the state Transportation Department is installing pylons on its next Dallas HOV project, on U.S. 75.
"I would certainly feel like your situation is set up to be more successful than what we've had in Dallas," Cooner said.
On the other hand, could funneling everyone who wants to enter or exit the managed lane into a quarter-mile section actually cause more accidents? Cooner said that's an ongoing debate in traffic design circles, one yet to be settled by reliable research.
The bible for U.S. highway design is called the green book, a thick manual published by the American Association of State Highway and Transportation Officials.
It says that on urban freeways, "through-traffic lanes should be 12 ft. wide." On freeways with at least six lanes, such as MoPac, the book says both shoulders should be at least 10 feet wide.
That won't be the case for certain sections of MoPac under the proposal. However, that is already the case near 35th and from Town Lake south to Loop 360 (Capital of Texas Highway). That section south of the river also has shoulders only a couple of feet wide.
The narrower lanes — 11 feet vs. 12 feet — are unlikely to cause any significant increase in accidents, several engineers say. But they could decrease capacity on the regular lanes, because drivers intuitively leave more space between their front bumper and the car in front when the lane is narrower.
The managed lanes could have a similar congesting effect on the inside regular lane, said Elizabeth Jones, an associate professor of civil engineering at the University of Nebraska.
"It's like being in that far right lane where people are entering and exiting," said Jones, a University of Texas graduate who specializes in transportation systems. "You get a little bit of turbulence there, so you don't get as much capacity."
Daigh says this $110 million project is a stopgap solution. A long-term approach, which might involve sinking MoPac belowground like Dallas' Central Expressway, would come much later.
"We cannot wait in this community 10 years to make some improvements" to MoPac, Daigh said. "This is not a silver bullet that will solve all of MoPac's problems. But it is a good step in the right direction."
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