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IH-30 Dallas HOV Lanes: An Inside Look

History

In 1990, construction began to create a new managed lanes system on IH-30 in Dallas, TX.  A High Occupancy Vehicle lane (HOV) would run for five miles into the downtown in the morning and three miles out of the city center for evening commuters.  Three scenarios were originally considered: 1) an exclusive HOV facility in the median, 2) a concurrent flow HOV facility on the inside shoulder, and 3) a contraflow HOV facility using a lane in the off-peak direction.  Because of bridge structures at different elevations and an inside shoulder that was not continuous, the contraflow lane was the only viable option.  The contraflow was possible because of a 70/30 tidal flow traffic split between peak and off-peak traffic, which allowed a lane to be taken from the off-peak traffic side without negatively affecting traffic flow in that direction.

The managed lanes facility was opened to the public in May, 1991 using a moveable concrete barrier.  The barrier is stored next to the median on both sides of the freeway (see graphic below).  During the morning commute into the city, the barrier on the off-peak side is moved out one lane plus the shoulder area by a Barrier Transfer Machine (BTM), and eligible vehicles are allowed to cross over to the other side of the freeway and run contraflow to traffic. When the peak period ends, the barrier is shifted back to the median and the lane is returned to concurrent flow traffic. This process is repeated on the other side of the freeway for the evening peak commute. By using a moveable barrier, IH-30 can be reconfigured on a daily basis for a fraction of the cost of building the same capacity (two additional lanes) using traditional construction methods, while opposing traffic lanes still benefit greatly from positive barrier protection.

When this moveable barrier “demonstration project” was implemented, it was believed that the B/C might be between 1.2 and 1.9.  In 2005, a review by FHWA found that the B/C was 28 : 1, as compared to building two general purpose lanes which would have had a B/C of 10 : 1.  Today there are over 200,000 vehicles per day using IH-30 in and out of the city center, with almost 10% of those vehicles moving through the HOV lanes.  The managed lanes have 2.6 passengers per vehicle, with up to 6,106 vans and carpools using the HOV lane daily.  200 buses per day run through the facility, and while the general purpose lanes average 27.5 mph during rush hour, the HOV lanes average 51.5 mph.  The moveable barrier contraflow lane, now in its 26th year of operation, was upgraded several years ago with two new BTMs.  The value of the HOV lane continues to make it the best option for this corridor, and while it is still considered to be an “interim project” there are no current plans to replace it.

Safety

Highway safety cannot be compromised by the implementation of a managed lanes facility.  Agencies must consider how the new lane configurations will affect the overall safety of the motoring public through the facility.  In September, 2003, the FHWA conducted a study of HOV corridors in Dallas to determine if there was a difference in crash statistics between barrier-separated HOV lanes and buffer-separated HOV lanes (buffer separated lanes have a paint buffer and/or plastic delineation to separate the managed lanes from the general purpose lanes, and access to the managed lanes is allowed only through specific ingress and egress locations.)  To collect data for the barrier-separated lanes, the FHWA chose to examine the moveable barrier facility on IH-30.  The study found that while the moveable barrier HOV facility did not show a statistical increase in accidents, two buffer-separated HOV facilities reported accident increases of 41% and 56%.  This major increase in accidents was attributed largely to the differential in speed between the HOV lane and the adjacent general purpose lane.  While speeds in the HOV facilities averaged between 52 and 66 mph, the adjacent GP lanes traveled at only 18 to 38 mph.

Traffic Control

Traffic control is an important part of opening, closing and reversing lanes in an HOV facility.  Unfortunately, it can also be a dangerous job.  To remove the HOV Operations Crew from harm’s way, the Dallas IH-30 facility uses an automated lane closure system called the SwiftGate from Versilis.  This system consists of a number of tapered gates that pivot from a stored to a deployed position to channel motorists into the correct traffic configuration.  The SwiftGate system can be remotely operated and monitored from traffic headquarters.