Active Traffic Management: Comprehension, Legibility, Distance, and Motorist Behavior in Response to Selected Variable Speed Limit and Lane Control Signing

Chapter 1. Introduction

ATM incorporates a collection of strategies that allows the dynamic management of recurrent and nonrecurrent congestion based on prevailing traffic conditions.⁽¹⁾ These strategies help to increase peak capacity, smooth traffic flows, and increase safety on busy, major highways. Some popular ATM approaches include VSL signs, LCSs (also called dynamic lane markings), and hard-shoulder running, which are all controlled by overhead CMSs.

Although approaches vary, ATM strategies and signing are deployed or in development in the United States, the Netherlands, Germany, United Kingdom, Denmark, Australia, Austria, New Zealand, Israel, Greece, and other parts of the world.^(1,2) This report describes research done on two particular types of ATM: VSL signing and LCSs. Previous research on VSL signs showed both safety benefits and efficiency improvements, although the evidence for efficiency improvements was less conclusive than desired.⁽²⁾ The safety benefits from VSL signs are most often associated with reducing rear-end collisions. This is not surprising because VSL signs are effective at reducing speed variability.⁽³⁾ In contrast, there have been relatively few studies on the impact of LCSs on driving behavior, safety, and throughput. However, one study involving a dynamic lane merge traffic control system that was deployed in Michigan indicated that this type of system could be helpful in reducing aggressive driving, increasing safety, and reducing delay at work zone lane closures.⁽⁴⁾ Other studies compared the use of graphic displays to their equivalent text messages and found that graphic displays could improve drivers' abilities to identify available lanes in a problem area and help comprehension for non-native language drivers.⁽⁵⁾

In the United Kingdom, the Transport Research Laboratory conducted driving simulation studies to support the development of a new approach to the design of managed motorway schemes. The approach is referred to as Managed Motorways-All-Lane Running (MM-ALR).⁽⁶⁾ The MM‑ALR approach employs gantry-mounted signs for speed and lane control similar to those used in the United States for ATM. The studies focused on examining drivers' understanding of the signs, as well as their driving behavior in a simulator. The studies generally found that drivers understood the signs and executed the appropriate behaviors. The simulations included simulated traffic, which appeared to affect the participant's compliance with the speed limits. Participants tended to exceed the speed limit more than 50 percent of the time under all of the conditions that were evaluated.⁽⁷⁾ The authors concluded that in busy traffic conditions, the speed of an individual driver was greatly influenced by the speed of surrounding traffic. Furthermore, some drivers may not have felt the need to maintain awareness of the current speed limit because it would influence their speed choice so little in busy traffic.

Although there is an increasing interest in VSL and LCS deployment in the United States, there are no design standards and a lack of empirical data on driver behavior as a function of sign design. At the time of this study, two LCS and VSL deployments in Washington and Minnesota used different approaches for sign structure and content. The two sites were using different symbols. In Minnesota, the per-lane CMSs were being added to existing sign structures and thus were in proximity to conventional highway signs. In Washington, the per-lane CMSs were installed on dedicated gantries, and additional CMS were used in conjunction with the per-lane signs. Both sites were operating under FHWA approvals to experiment with novel ATM signs (see chapter 1, section 1A.10, Manual on Uniform Traffic Control Devices (MUTCD), regarding experimentation).⁽⁸⁾

This report describes four studies (one in a laboratory setting, one in a field setting, and two in a driving simulator) in which two particular ATM approaches were researched: VSLs and LCSs. Chapter 2 describes a laboratory study in which participants viewed a series of lane control scenes from the perspective of a driver. Participants described what they thought the signs were intended to mean and what they would do in response to the signs. Subsequently, participants were asked which sign content alternatives they preferred. Chapter 3 describes a field test in which the legibility distance of a selected subset of sign alternatives was assessed. Chapters 4 and 5 describe experiments conducted in FHWA's Highway Driving Simulator to examine how drivers might behave in response to various scenarios in which LCSs and VSLs were employed. The experiments described in chapters 4 and 5 examined driver lane choice, speed, and eye-glance behavior in a dynamic environment in response to the same signs and scenarios examined in a static environment in chapter 3. Chapter 6 synthesizes the report and summarizes the important contributions made by this ATM study. The studies reported here were intended to contribute to the development of guidelines for consistent and effective ATM signing.