Variable speed limit (VSL) systems have been used in a number of countries, particularly in Europe, as a method to improve flow and increase safety. VSLs use detectors to collect data on current traffic and/or weather conditions. Posted speed limits are then dynamically updated to reflect the conditions that motorists are actually experiencing. Presenting drivers with speed limits that are appropriate for current conditions may reduce speed variance, a concept sometimes called speed harmonization. If properly designed, VSL systems have been shown to reduce crash occurrence and can also reduce system travel time through increased uniformity in traffic speeds. High-volume urban work zones tend to be prone to congestion and safety problems, and VSLs may be one way to ameliorate these issues. VSLs were recently installed at a high-volume, congested urban work zone located on I-495 (the Capital Beltway, hereinafter the Beltway) between the Springfield Interchange in Springfield, Virginia, and the Virginia-Maryland state line on the Woodrow Wilson Memorial Bridge. VSL signs were activated in late July 2008, but initial evaluations of the system showed inconclusive effects. Changing site conditions made a direct before-and-after evaluation of the system deployed in the field problematic, and some problems with the control algorithm were also noted. Given the difficulties in evaluating the system deployed in the field, a calibrated simulation of the site was constructed to assess the effects of the VSL system on traffic operations and safety surrogate measures. The simulation platform also provided an opportunity to examine a number of system configurations to assess how changes in system design and driver behavior might affect a variety of measures. The results indicated that the VSL could create substantial improvements in traffic operations provided the demand did not exceed capacity by too large a margin. The location of the VSL signs played an important role in operational performance. The study recommends that the Virginia Department of Transportation continue to pursue this technology but carefully scrutinize algorithm design and VSL sign placement. Further, a cost/benefit analysis indicated that VSLs may be most appropriate for long-term applications.

There has been a considerable increase in the amount of construction work on the U.S. national highways in recent years. This is a direct result of the current transportation policies emphasis on the maintenance of existing facilities rather than on building new ones. But with the increase in the amount of traffic on the interstates, the safety at the construction sites is getting worsened. Variable speed limit (VSL) systems are a kind of intelligent transportation systems (ITS) technology and they try to increase safety by posting the maximum or minimum speed limits. The speeds to be posted on the VSL signs are calculated manually or with the help of algorithms. This study is an attempt to evaluate the performance of various speeds at work zones and to develop VSL control logics that would calculate suitable speeds for different traffic conditions. The research is conducted by simulating a postulated test-bed network and then validating the results by simulating a real world work zone site. The study uses a microscopic simulation model - VISSIM. The study developed a safety surrogate measure, minimum safety distance equation (MSDE), to quantify safety. Travel times have primarily been used to quantify mobility. Since most of the existing VSL control logics consider only the traffic advisory speed or average traffic speed, an attempt was made to develop a logic that would consider both safety and mobility in calculating speeds. The results of simulating the postulated network indicated that the performance of speeds varied with traffic demand volumes and compliance rate conditions. In general an increase in speed decreased safety, but improved travel time. Surprisingly at low volume and low compliance conditions an increase in speed also improved safety. In order to find the speeds that provide optimal measures of safety and mobility a normalization procedure that combines travel time and safety measures is used. The performance of the logics is mixed. In general logic 3 (that considers both safety and mobility measures to calculate VSL speed) performed better than logics 1 and 2 (that use average traffic speed). However logics 1 and 2 performed better than Logic 3 for oversaturated conditions and for low volume and low compliance conditions. Statistical analysis was performed to substantiate the results of the base cases and the logics. The performance of the VSL system was also evaluated by simulating a real world work zone site at Covington, Virginia. The results were found to be consistent with those of the postulated network.

The project is the result of a joint public-private venture led by the Michigan Department of Transportation (MDOT) and includes the participation of the Michigan Department of State Police (MSP), Michigan State University (MSU), and NES Worksafe (Worksafe). International Road Dynamics (IRD) was also actively involved in technology development and deployment through Worksafe. The system that was tested and deployed experienced problems that might be expected of a prototype as opposed to a fully tested and refined system. The most important modifications include: better communications with individual trailers and between trailers, better and more flexible vehicle sensors, the ability to monitor (or use) individual vehicle speeds, better and more flexible capability in establishing the algorithms for setting limits, and easier to follow procedures and rules for using the system. As evaluated, the system suffers from a lack of real portability (due to the reasons just mentioned) which limit its ease of use in the often-restricted work zone environment. The system had relatively minor impacts in the work zone in which it was used. As it turned out, the topography of the area and the existence of the ramps and bridges associated with a freeway-to-freeway interchange resulted in significant restrictions being placed on the speed limits that could be used. In addition, the presence of the ramps and the work activity resulted in relatively low speeds under many conditions. These limitations notwithstanding, there were positive effects on average speeds through the VSL deployment area (increased) and travel time (decreased). Effects on the 85th percentile speed and speed variance were either undetectable or inconsistent. The percentage of vehicles exceeding certain thresholds (e.g., 60 mph) did, however, decrease when the system was in operation. Apart from the traffic-related MOEs, the presence of enforcement personnel in the deployment area appeared to have no additional or interactive effect. Finally, an anecdotal review of the crashes in the area showed that most crashes were rear-end collisions and none appeared to be directly associated with the deployment of the system; the system certainly did not seem to create additional safety problems in the deployment areas. Despite the paucity of usable data, it is also seems clear that VSL systems will have different applicability in different types of work zone situations. The conclusion is drawn that VSL systems will have more utility in longer and "simpler" work zones. For example, long zones with short areas of actual work. These limitations notwithstanding, it was also seen that the VSL system can present far more credible information to the motorist, responding to both day-to-day changes in congestion as well as significant changes as motorists go through a given zone.

Speeding is the number one road safety problem in a large number of OECD/ECMT countries. It is responsible for around one third of the current, unacceptably high levels of road fatalities. Speeding has an impact not only on accidents but also on the ...

Technological advances allow speed limits to be changed in real time based on actual roadway conditions. There may be potential advantages to using variable speed limits in Utah, such as for traffic control during construction where the speed limit may be lowered adjacent to actual construction activities versus at other times or locations within an established work zone. This research will test the compliance and derive potential advantages from the use of variable speed limit equipment.