Variable speed limits (VSLs) have had numerous applications over several decades with increasing interest in the last several years. The basic premise of VSLs is that in some situations the normally posted regulatory speed limit should vary dynamically with conditions encountered on the roadway (e.g., inclement weather, work zones, school zones) and/or congestion that may be present. This is in response to the often-stated argument that some invariant static limits are perceived to be “unreasonable” to the average motorist. In turn, speed limits that are perceived to be unreasonable can lead to low speed-limit compliance rates, high variance in vehicle speeds, and unsafe conditions. With VSL, the hypothesis is that motorists will respond “better” to realistic speed limits, resulting in higher compliance, lower speed variance, and safer conditions. From the motorist’s perspective, one of the stereotypical situations where unrealistic static limits are often encountered is in construction/work zones–an example being a very long marked work zone (with a low statutory limit) where no actual construction activity is encountered until miles after the start of the zone or such activity occurs only for short sections within the zone. In this context, the US Department of Transportation’s (USDOT) Federal Highway Administration (FHWA) solicited applications for field tests of VSL systems in work zones. Michigan was one of three states chosen to undertake the field tests. The project reported on here 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. An extensive literature review is not provided here as reviews have already been done by others and are generally available. Principal among these is that done by researchers at Northwestern University under the auspices of National Cooperative Highway Research Program project 3-59, Assessment of Variable Speed Limit Implementation Issues, wherein experience both in the US and abroad is catalogued and a comprehensive list of vendors of various system components is compiled and presented. The basic objectives of this project (as articulated by the FHWA) were to design and deploy a viable VSL system in a work zone and evaluate the extent to which: speed limit compliance is affected; the credibility of the speed limits is increased; safety is improved; and traffic flow is improved. In this context, VSL is a field application of intelligent transportation system (ITS) technology. The VSL system used in Michigan was developed and deployed by Worksafe/IRD with direction from MDOT and MSU. The system was “bench tested” by Worksafe/IRD, initially field-tested on a local road (under direction of MSU and MDOT) to confirm that the system was basically operating as designed (and required), and then deployed during the summer (2002) in a work zone on I-96 south and west of Lansing, Michigan. There were four separate and different deployments within the larger (~18-mile) work zone during which operational data were collected. The data were then analyzed in the context of the operations of the VSL system itself as well as its effects on traffic flow. This report contains the following components: a description of the partnership in place for the demonstration project; information about the legality of VSL in Michigan; a brief description of the VSL system deployed in Michigan; a description of the pre-deployment and testing of the system; information about the overall deployment site; basic system operations; and system effectiveness. The report is concluded with discussion and comments regarding further applications of VSL in work zones. Throughout the report, issues and problems encountered are discussed with resolutions noted. In some instances, references are made to separate documents (e.g., the IRD manual for the system deployed) which are not a formal part of this report. These are available from the original authors of the various documents and reports. (Author/publisher)
Abstract