Effects of route guidance variable message signs (VMS) on driver behaviour

Abstract

Effects of route guidance Variable Message Signs (VMS) on speed and route choice were investigated in a field study on two sites on motorways. Two VMS were used in the study, which displayed information about a closed road section downstream on the motorways and recommendations for alternative routes. Route choice, speed and braking behaviour were compared between vehicles approaching the VMS while they displayed messages and while they were left blank without message. There was high compliance with the messages on the VMS. About every fifth vehicle changed route choice according to the recommendation, and almost none drove as far as the closed road section. Speed measurements of 3342 vehicles showed large speed reductions, and video observations showed that large proportions of vehicles braked while approaching the VMS. The speed reductions and braking manoeuvres can partly be attributed to attention overload or distraction due to the information on the VMS. However, a proportion of the speed reductions was due to chain reactions where one vehicle braked and forced the following vehicles to brake or change lanes in order to avoid collisions. Safety problems may result directly from distraction, or indirectly from the reactions of the drivers to the distraction.

Introduction

The aim of this study is to analyze effects of Variable Message Signs (VMS) on route choice and safety. Two VMS on motorways outside Oslo were studied, which showed messages about a closed road section downstream on the motorway, and recommendations for an alternative route. Research on VMS has shown that drivers generally welcome information about incidents or congestions and suggestions for alternative routes. However, the proportion of drivers actually changing route choice varies considerably, and is seldom over 40%. In a Swedish study (Davidsson & Taylor, 2003) between 6% and 41% followed a recommendation on an alternative route in order to avoid congestion. Ramsy and Luk (1997) found that the proportion of diverting vehicles increased by 30% when a road blocking was announced. In the study of Lindkvist (1995) between 5% and 25% chose an alternative route, and Cummings (1994) found that only 4–7% of all drivers diverted. In the study of Chatterjee, Hounsell, Firmin, and Bonsall (2000) only one third of all drivers noticed messages about network problems, and few of these drivers diverted. While the aim of congestion information is an optimization of traffic flows where 100% compliance with the suggestions would not be desirable, the messages on the VMS in the present study require close to 100% to achieve maximum effectiveness.

There are several possible explanations for the low diversion rates. Drivers may for example overlook the messages as in the study of Chatterjee et al. (2000), they may not understand or not trust the messages, not find the recommended alternative route, or suspect that the alternative route takes more time (Kronborg, 2001). Text messages impose high attention demands on drivers and require longer reading distances, compared to symbols or pictograms (Nuttall, Ginsburg, & Beerlage, 1998). Pictograms are preferred by drivers, although even pictograms are not always understood (Rämä, Schirokoff, & Luoma, 2004). Several studies have shown that VMS impose higher attention demands on drivers than fixed signs (Anttila et al., 2000, Rämä et al., 1999). The readability and comprehensibility is additionally affected by the contents of the message. Whereas information should ideally be familiar and standardized (Proffitt & Wade, 1998), rerouting information that is presented on VMS is usually both unfamiliar and unstandardized.

Route guidance VMS have primarily the aim to improve traffic performance, not to improve safety. They may however affect safety. Vaa, Gelau, Penttinen, and Spyroupolou (2006) have classified ITS systems according to whether they limit or enhance the space of behaviour opportunities. Route guidance systems are systems that enhance the space of behaviour opportunities, and may increase the frequency of distractions. Drivers may be distracted by VMS because they need to allocate attention to the VMS in order to read and understand the message, and to draw conclusions about their own behaviour (Hancock and Verwey, 1997, Rasmussen, 1983). Distraction may also be due to the more advanced technology as suggested by the study of Rämä et al. (1999). At the same time they have to pay attention to the driving task. If there are conflicting attention demands, drivers may reduce the amount of attention they pay to the VMS or to the driving task. If they pay less attention to driving, adverse effects on safety may result (De Waard & Brookhuis, 1997). Alternatively, drivers may take actions which decrease attention demands, for example reduce speed (Krammes and Glascock, 1992, Sagberg, 2003). Speed reductions lower the amount of attention which has to paid to the driving task, all else being equal (De Waard, 1996), but they may, especially when they are unpredictable for other drivers, lead to conflicts with other vehicles, short headways, or to risky driving manoeuvres like lane changes (Abdulhai and Look, 2003, Erke and Gottlieb, 1980).