Comparisons with Results of Other Studies

The findings of this study are comparable to previous research for several of the sites, and go beyond previously available knowledge on benefits for others. The following sections outline some of the previous research and show how the findings of this study compare with other studies.

Two system types are identified for comparison. The first is the type of system that physically controls traffic conditions such as the Michigan Dynamic Lane Merge System. The second is more of a monitoring and information dissemination system that provides motorists with real-time information so that they can make informed route choice decisions, such as those deployed in Arkansas, North Carolina, and Texas. Both have an element of controlling traffic to improve the operational performance of the work zone.

Previous lane merge studies showed significant reductions in aggressive maneuvers at the work zone taper similar to the findings of this study. For sites with the potential for longer queues, agencies have implemented and studied the late merge system, which also showed direct benefits. The traffic monitoring systems have proven more difficult for measuring quantified benefits, as the benefits are typically indirect in that these systems generally provide enhanced information to motorists. The systems that display alternate route guidance have been implemented to a greater degree recently, including those studied for this project.

Enforceable Merge Systems

Michigan DOT undertook several studies in the recent past to evaluate the effectiveness of the Dynamic Lane Merge System. The system merges traffic early in locations where queue spillback is at acceptable levels. MDOT cited an effectiveness range of 3,000 to 3,500 vehicles per hour for the three-to-two merge situation, and 2,000 to 3,000 vehicles per hour for the two-to-one merge situation. Queuing will occur in both situations and is needed to warrant use of the system since queue conditions create a potential for forced merges. For the I-94 deployment[7] (three-to-two merge), the average number of aggressive maneuvers during the peak hour decreased from 2.88 to 0.55. For a two-to-one deployment on M-53 in Grand Rapids, Michigan, the average number of aggressive maneuvers decreased from 68.0 to 32.0 during the morning peak period, and decreased from 38.0 to 9.0 during the afternoon peak period. The percent reduction in aggressive maneuvers for the Michigan site in this study was higher than previous studies; however, the total number of observed aggressive maneuvers was lower.

For higher traffic locations, agencies have successfully used the Late Merge System to smooth merges at the work zone taper. The Late Merge System allows traffic to use both lanes on one approach and advises motorists via dynamic message signs to take turns merging from each lane. Minnesota DOT found that discontinuous lane usage increased to 60 percent at one sensor location on I-494.[8] While MnDOT was not able to perform a true before and after comparison (temporary traffic control conditions both with and without ITS), evaluators observed minimal queues during the course of the study. Research has shown that the late merge system is better for higher traffic levels and where the availability of queue storage is low, while the early merge system is a tool for lower demand, higher queue storage locations. However, a system of message signs and sensors that can adjust automatically between the early merge and late merge concept may be most beneficial to owner-agencies. This type of active merge system has been used in Minnesota.

Traffic Information Systems

Some agencies have tested and used mobile traffic monitoring and management systems to provide real-time information to motorists. These systems display information about work zone conditions, but can also play an important role in alleviating traffic congestion due to incidents. It is often more difficult to evaluate the benefits of these types of systems in a quantifiable way, especially for metrics such as safety. For example, a system may provide advance warning of queued conditions to reduce speed variability and the potential for rear end collisions. But, with many common limitations in evaluating crash records for safety performance, agencies may find it difficult to quantify the benefits. Such benefits are needed to build support from decision makers to continue use of such systems.

Some systems are also designed to provide information but with one direct outcome in mind, such as to reduce speed at a work zone to improve the safety performance of the work zone. A Midwest Smart Work Zone Initiative Study of a speed monitoring and display system found a significant reduction in speed (5 mph) near the work zone taper. Three other deployments along I-80 near Lincoln showed similar results. The study team observed a 3 to 4 mph reduction in mean speed, a 2 to 7 mph reduction in 85th percentile speed, and about 20 to 40 percent increase in vehicles complying with the speed limit.[9]

Some studies have tested diversion around work zones based on real-time information. Some systems actively divert traffic by providing guidance to motorists, while others provide general delay information and allow motorists to make route decisions. The limitation of the latter concept is that likely only motorists familiar to the area will divert without the direct guidance to do so. For areas with mainly through traffic, this concept should be considered in the design stages to ensure the intended outcome is realized. A study of the effectiveness of an Automated Work Zone Information System on Interstate 5 in California showed diversion rates of 9 percent to 12 percent based on condition information.[10] Similar studies in Nebraska and Kentucky showed very little diversion based on general condition information. While the traffic makeup is unknown for the sites mentioned (commuter versus through trips), specific guidance for motorists on when to divert to alternate routes will, under the appropriate conditions, have a better result. For example, a study of a North Carolina Smart Work Zone deployment found that, "…alternate route usage is increased in the range of 10 to 15 percent with the presence of a Smart Work Zone that provides specific information about delays and alternate routes."[11] In general, practitioners in North Carolina have observed, "…some increase in usage of alternate routes," across their various deployments of work zone ITS.[12]

Additional Comparative Data

A recent FHWA review of work zone ITS benefits[13] produced the following documented results:

• Between 50 percent and 85 percent of drivers surveyed said that they changed their route at least sometimes in response to travel time, delay, or alternate route messages provided by work zone ITS.
• Reductions in queue lengths from 56 percent to 60 percent are possible, with simulations indicating system-wide reductions in total delay may range from 41 percent to 75 percent.
• Speed monitoring displays (SMD) reduce speeds in work zones by 4–6 mph. One study found a 20–40 percent reduction in vehicles traveling 10 mph or more over the speed limit when SMD were used.

The results of this study are comparable to the other studies referenced herein, with some additional insights into deployment and assessment of ITS for work zone applications. The results from this study for the lane merge system is comparable to that of past evaluation projects such as those performed by Minnesota DOT and Michigan DOT, while the benefits of providing alternate route information are further substantiated through this study. Some previous studies focused more on changes in performance measures and statistical analyses related to metrics such as speed. Additionally, this study provides further evidence of the general benefits of properly planned and design work zone ITS deployments.

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