I-35 in Waco, Texas

Work Zone and System Description

In October 2006, The Texas Department of Transportation (TxDOT) implemented an ITS system in a construction work zone on I-35, south of Waco, in Hillsboro County. The purpose of the system was to provide motorists with real-time information on downstream conditions and to provide alternate route guidance during times of heavy mainline congestion. TxDOT sought to warn motorists of speed variability issues and to lessen traffic delays caused by capacity reductions and rubber-necking in the work zone.

TxDOT designed the system as shown in Figure 3. The system consisted of six microwave sensors, six message boards, a central processing and communications unit, and three wireless closed circuit video cameras. TxDOT procured the system through the prime construction contractor.

Figure 3. I-35 Work Zone and Signed Alternate Routes (Taken from TxDOT's Website)

Based on predetermined speed and occupancy thresholds, the system provided real-time delay information and recommended alternate routes via DMS. TxDOT operators also monitored traffic conditions in the work zone through the use of three wireless closed circuit video cameras.

The primary function of the system was to monitor the project work zone and automatically provide alternate route advisory information to the traveling public based on significant travel times through the work zone. A secondary benefit of the system was congestion management of non-recurring traffic conditions because of high traffic volumes, weather, and incidents.

Focus of the Evaluation

The main objective for this deployment was to reduce demand and congestion through active diversion of traffic approaching the work zone. Diversion was encouraged by providing real-time information and route guidance to motorists en-route to warn them of downstream conditions, and by providing trip planning information via a website.

The study team mainly focused data collection efforts on testing the level of diversion around the work zone during periods of congestion. The study team collected ramp and mainline volume, speed, and occupancy data from microwave sensors placed at each diversion point to quantify the level of ramp usage for "with" and "without" ITS conditions. The study team archived data from October 3, 2006 through February 4, 2007.

Findings

The evaluation showed that during times of very heavy congestion, motorists will follow the diversion guidance posted on message boards. Large percentages of traffic diverted on several occasions when the system recommended the alternate route. The study team found 1 to 28 percent reduction in mainline traffic volume (with an average of 10 percent reduction) during congested periods, lessening the demand for restricted mainline capacity. These results are based on 20 observation periods during which the system actively diverted traffic due to congestion from construction or incidents. The study team was not able to determine the impact of the diversion on mainline delays/travel times because the system had only two sensors collecting speeds (spot detection) over a short distance, and to determine travel times across the entire route (from the diversion point through the end of the work zone) would have required more sensors to provide better accuracy.

Key Finding: During major incidents or high construction impact periods combined with high demand, the system diverted an average of 10% of mainline traffic to alternate routes. Diversion was as high as 28%.

TxDOT signed appropriate alternate routes for use during periods of diversion, but did not monitor congestion on these alternate routes. It also should be noted that for the 20 times that the system actively diverted traffic, incidents were more typically the cause of congestion than construction. Congestion due to construction was not a long term problem.

The system detected congestion and displayed appropriate messages, although the minimum display time for diversion messages was likely too short in some cases. The system posted travel times for conditions at or near free flow travel times, "Slow traffic ahead" and similar messages when speeds dropped, and diversion messages when occupancy met the appropriate threshold. When the system posted messages recommending the signed alternate routes, large percentages of traffic diverted. Major incidents during heavy traffic periods (such as holiday weekends) were a main cause for active diversion more than typical construction activity and lane closures. In these cases, the system detected and reacted to the congestion caused by incidents just as it would for congestion caused by construction activity. It should be noted that this is a rural area with a large number of through trips and drivers unfamiliar with the area.

Key Finding: The real-time information system effectively diverted traffic to specific, signed alternate routes during times of significant congestion.

Tips and Lessons Learned

The study team identified several lessons learned that can benefit those interested in deploying similar systems.

Begin work on the deployment at the early concept stages of the planning process. The deployment at this site was delayed due to lead time in procuring the system. To achieve the maximum benefit, ITS should be operational prior to any lane restrictions.

In design and implementation of ITS for work zone applications, agencies should involve the construction contractor as early as possible and to the fullest extent possible. TxDOT experienced unanticipated delays in modifying the contract to include the ITS system. Agencies risk not achieving their goals if the system is viewed as a "pass through" to a vendor and not the responsibility of the contractor.

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