Variable Speed Limits: Safety and Operational Impacts of a Candidate Control Strategy for Freeway Applications

Abstract

Variable Speed Limit Sign (VSLS) systems enable transportation managers to dynamically change the posted speed limit in response to prevailing traffic and/or weather conditions. Although VSLS have been implemented in a limited number of jurisdictions throughout the world there is currently very limited documentation describing the quantitative safety and operational impacts. Furthermore, the impacts reported are primarily from systems in Europe, and may not be directly transferable to other jurisdictions, such as North America. This paper presents the results of an evaluation of a candidate VSLS system for an urban freeway in Toronto, Canada. The evaluation was conducted using a microscopic simulation model combined with a categorical crash potential model for estimating safety impacts. INTR ARIABLE Speed Limit Sign (VSLS) systems consist of dynamic message signs (DMS) deployed along a roadway and connected via a communication system to a traffic management centre. The VSLS are used to display a regulatory or advisory speed limit. Unlike typical static speed signs, the VSLS system enables transportation system managers to dynamically post a speed limit that is appropriate for current traffic, weather, or other conditions. VSLS are thought to improve safety and reduce driver stress while improving traffic flow and travel times [1]. Worldwide, VSLS systems have been deployed in a limited number of jurisdictions including the UK, the Netherlands, the USA, Germany, Australia, and New Zealand. Benefits have been reported from empirical studies in terms of safety with reduced collisions [2, 3] and in terms of improved traffic flow perceived by the driver [4]. Although in general, benefits have been recognized, most of the empirical studies to date are limited by one or more of the following: ODUCTION • Lack of control of important influencing factors such as traffic volumes, degree of enforcement and compliance, etc. • Empirical benefits reported largely in terms of qualitative evidence. • Transferability of results to other jurisdictions (ie. Europe to North America).

Towards Formal Verification of Freeway Traffic Control

“…Intelligent speed adaptation [10] and variable speed limit sign systems [1] have increasingly gained attention as a means to increase road traffic safety. Related research in these areas, however, (i) focuses on experiments and simulations of traffic behavior [4], [5] and models for determining optimal speed limits [11], (ii) shows the effectiveness of speed adaptation in terms of reducing casualties [1], [12], homogenizing speed [3], increasing compliance with speed limits [10], as well as (iii) discusses impacts on travel time and throughput. We, instead, investigate constraints that implementations of such a system must respect and verify the safety that can be guaranteed under these constraints.…”

“…A typical application of intelligent speed adaptation with variable speed limits is to lower and homogenize speed in the area of traffic incidents [1]. For example, Lu et al [8] use variable speed limit control to maximize bottleneck flow in an area of a lane drop, such as encountered when lanes merge or lanes are closed due to road work.…”

“…Typically, variable speed limits are used to lower and homogenize speed in the area of traffic incidents [1], which can be static (i.e., at a geographically fixed position, e.g., construction sites) or moving (e.g., traffic jams, wrong-way drivers). In order to be effective, a variable speed limit therefore must be activated geographically in front of such an incident (from the viewpoint of cars approaching an incident,).…”

“…The available control options comprise, for instance, variable speed limits, ramp metering, lane closures, detours, arterial traffic light control, and warning signs displaying traffic incident information (e.g., traffic jams, construction sites, or driving conditions). A number of theoretical and experimental results have shown that such global highway and freeway traffic control increases safety [1], [2], homogenizes traffic flow [3], and may increase the flow during peak periods [4], [5]. Today's highway and freeway traffic control is centralized in traffic centers (e.g., on a per state level) with little direct influence on the behavior of cars, making it an open-loop control system.…”

Towards a Safe and Efficient Motorway System: Literature Review on Variable Speed Limits

“…The control logic, threshold values and detector-controller layout applied to speed limiters are considered to a partially determinant factor for success of VSL (Li et al, 2015;Allaby et al, 2007). Li et al (2014) proposed some modifications to detector location and control logic for the logic tree based VSL control algorithm proposed by Allaby et al (2007) which is implemented in a critical section of Auckland Motorway using AIMSUN micro-simulator. They concluded that the performance of VSL is highly dependent on the control logic embedded and on the location of the VSL detectors.…”