Road Travel Time Information on VMS and Traffic Congestion

Brigitte Cambon de Lavalette*, Charles Tijus**, Sébastien Poitrenaud¨¨, Christine Leproux***LPC-INRETS France

**Laboratoire Cognition et Usages, Université de Paris 8, France

ABSTRACT

Some years ago, progress in road telematicshad made it possible to introduce time travelinformation in real time on VMS devices.Such information can lead road users to avoidcongested areas as well as to waiting in abetter condition for the end of the congestion.In a way, they increase congestion, in anotherway, they reduce it. Our research aimed atexplaining the reasons why some peoplebehave in a way and another one in. Weassumed that travel time messages may beinterpreted from several points of view,depending on driver knowledges in memory,experience and will, and as a result, thedecision taken.An enquiry on a group of drivers aimed atanalysing mental activity involved in decisiontaking process will be presented.

1. INTRODUCTION

Traffic saturation in urban areas is a never endingproblem for traffic managers. Though urbanexpressways were originally conceived for easingtransit, little by little they have also become thescenes of daily congestions. This state of things is notwithout consequences for the quality of life in thecities, on account of the loss of time involved or eventhe irritation expressway users may feel when theyare trapped in traffic jams.Progress in road telematics has introduced variablemessage signs (VMS's) which should constitute aprecious aid to traffic engineering. It can provide away in regulating traffic stream as well as to driverswho wish to know how long it needs to reach theirdestination. These are devices designed to improvethe flow of traffic according to a self-regulatingprocess: the driving time between two points isdisplayed in real time, drivers can evaluate the stateof traffic and may choose another less encumberedroute if need be.

However, the semantics of VMS messages are clearlydifferent from traditional signposts which do notprovide for announcing transitory events in real time.How do drivers interpret these messages, what arethey lead to decise from them?At the beginning, when travel times appear on VMSin the road network, they were supposed to be used toavoid congested areas. In fact, according to theopinion polls commissioned by the transportationauthorities, although drivers are for the most part(98%), glad to benefit from this new service, most ofthe time, they are no more likely to change theiritinerary when the expressway is crowded (Ref. 1). Inother words, the VMS’s are probably useful to usersbecause they have expressed their satisfaction withthem, but the use they seem to make of them isseemingly not the use for which they were intended.As a result, travel time information affects trafficcongestion at the opposite of expected traffic state.Our research was aimed at understand the reasonswhich in the driver mental process can explain suchan effect. At the same time, we observed the road userneeds according to time travel information.

The device which the use we observed is put on anexpressway that circles Paris. It is a ring road 37 km.long, called the “Boulevard Périphérique”, thatconnects the 32 “Gates” into the city. The VMS’sstationed approximately every 500 metres indicatetwo destinations (which are “gateways” into the city),followed by the driving time it will take to get there(see figure 1). The system which produces thesemessages functions as follows. Sensors embedded inthe carriageway calculate several variables in realtime: traffic flow, speed and a representation of trafficdensity. The sensors are spaced approximately 500metres apart, as are the signposts; and measurementsare taken every minute.

2. HOW DO DRIVERS INTERPRET TRAVELTIME MESSAGES ?

Traffic measurements and observations of behaviourhave already been carried out to evaluate the impactof variable message signs on driver behaviour (2).The results reveal reactions which are difficult to

interpret within the generally adopted framework ofdriver rationality. We therefore need to understandthe mental process involved in reading the messagesbefore the driver takes any decisions and the types of

Figure 1. Variable Message Signs device on theexpressway around Paris (Photo. INRETS)

reasoning drivers use when deciding whether tocontinue on or to leave a saturated route. In view ofthe mental process involved, the decisional processthat precedes decision making may involve severalmotives and inferences that are probably morecomplex than those which are immediately apparent.Two types of questions are considered in turn: first,what knowledge is involved in interpreting messagesand determines the meaning ascribed to them, andsecond what intentions may inform the decision infavour of one or other alternative, namely keeping tothe same route or changing it.

2.1. KNOWLEDGE INVOLVED IN MESSAGEUNDERSTANDING

Obviously, one factor involved in messageunderstanding is knowledge required to interpret whatit is meaning at. Time travel messages arealphanumerical and brief. They state a location and aduration, which means that the travel time betweenthe sign and the location which is downstream on thesame road is currently the number of minutesindicated on the device. The elliptical nature of themessages assumes that drivers already possess acertain amount of knowledge, if only topographical,

about the area and are also able to represent andprocess this knowledge, which implies diversifiedmental process that uses processes whose paths mustbe mapped out in order for the outcome to beunderstandable. Then, knowledge refer to all thememorized information involved in the interpretationof the message but also some properties of thisknowledge, for example the reliability of travel timemeasurement process. It is easy to see that it isimportant to trust the reliability of the measurementdevice in order to base an action on it. The actiondrivers decide to take in response to messagesdepends on how reliable they think they are. Driversmust therefore understand that the displayed journeytime is the journey time for the journey they willmake. The main value of the time displayed onvariable message signs is that it is the result of a realtime assessment and gives an instantaneous picture ofthe situation at the moment in question. Drivers needto understand that the assessment is output by asystem that collects data on traffic conditions at theinstant in question. It is therefore necessary to haveconstructed a mental representation of a system that isable to produce reliable current traffic data.

We can identify various types of knowledge in thiscontext. For example, when the message states atravel time longer than normal, a driver may infer thepresence of congestion. However, a driver who doesnot know the normal time travel is not able tointerpret the information correctly because he or shelacks an adequate mental model. A driver who knowsthe locality and the normal journey time but does notknow an alternative route has an accuraterepresentation of the content of the message but doesnot possess the appropriate response in order tomodify his or her behaviour. Lastly, a driver whodoes not have a satisfactory mental model of theoperation of the system can interpret one of the itemsof information in a way other than that which isintended. For exemple, in Paris, where this researchtook place, the message « Porte de Bercy 5 min »could thus be taken as meaning « I passed the Portede Bercy 5 min ago » or « The displayed journey timeis short because they want motorists to use this routerather than another ». And then, if he ignores howthe message gets to the the sign, he can assume thatthe real time is now different.Briefly, knowledge we considered were thefollowing :

• Knowledge concerning the typonomy andtypology of the locations.

• Knowledge of usual time travel for thedestination mentioned

• Knowledge of alternative routes (mentalmap)

• Trust in the reliability of the messages

2.2. DRIVER INTENTION CONCERNINGSPEEDNESS

Traffic counts have revealed that time travelinformation does not lead to any specific routechanges. Drivers who are faced with the choice ofleaving a saturated motorway in order to use a freelyflowing alternative route prefer to stay on thesaturated road, even though they know how muchtime they will lose as a result. This suggests that theintention of saving travel time, which is very oftenascribed to drivers, is not all that dominant.Knowledge is certainly necessary, but it in notenough to induce action. It seems to us that for adriver to use his knowledge, the desire to reach hisdestination as rapidly as possible must beaccompanied by willingness and a capability to go byanother route.

The implicit model (3) of the driver which is designedinto the VMS system,, is that driving is a kind of“wanting to save time”, to go as quickly as possible,faster than one can go by public transportation forexample. But the extent to which this assumption isvalid is not known, despite the fact that it is acommonly held belief. It seems self-evident that themore a person wants to reach his destination asquickly as possible, the more he will avoid heavytraffic. There is a need to verify that the subjectsquestioned on this point, aside from what they feelwhen they are actually in a traffic jam, really do havethe intention of “saving time”. It is then easy to seewhy it is important to verify this point with thesubjects chosen.

It was apparent that, at an earlier stage than individualknowledge about the messages, drivers’ intentionswith regard to organizing their trips should also play apart in the decisions they take: what are their

priorities when making a journey? We haveconsidered two types of intentions:

• preserving one’s comfort : keeping currentaction, avoiding to searchand to reach foranother route

• reaching one’s destination as rapidly aspossible and saving time

2.3. HYPOTHESISThen, it could be assumed several categories of users.Here is an example: the driver’s intention is tocomplete the journey quickly; with regard toknowledge the driver knows the location of theplaces, the usual journey times, but does not know analternative route; with regard to reliability, the drivertrusts the message. Another possible category couldbe: the driver’s intention is to preserve his or hercomfort; with regard to knowledge the driver knowsthe location of the places, the usual journey times andan alternative route; with regard to reliability thedriver trusts the information.

Analysis of the inferences (4) that are generated bythe messages is of interest on the following grounds:the driver’s behaviour is not seen as the result of asingle internal variable but as the outcome of acomplex process made up of several successiveanalyses. On the basis of intentional forms andknowledge we can develop a taxonomy of thepractices generated by messages. This is relevant notonly to the regulation of transport but also to the moregeneral attempt to find processes for analyzing therationality of action.

To model this behaviour we have used Stone’s model[5], which is based on knowledge networks. Thisestablishes a taxonomy of the practices generated bythe messages, which takes account of the user’sknowledge of the system andmessage content (fig.2)..

Figure 2. The whole set of properties that can definedrivers as being a decomposition tree of actions,knowledge about routes, intention and motivs ofchoice for using automobile and for taking the theexpressway .

2. METHODOLOGY AND RESULTS

The participants were told that they were beinginterviewed in order to obtain a description of theroute they take to get to work (6). The interviewswith the driver-participants were conducted in such away as to gather information on the followingthemes.

1. The choice of the automobile over othermeans of transportation, the importance of “timepressure” in this choice and the reasons for using theboulevard peripherique (BP).

2. An account of the most recent actualitinerary taken, the objective of which was to gatherthe route the driver actually takes and the actualdecisions he makes.

3. Evocation of the different routes taken, inorder to get the participant to spontaneously revealthe importance of VMS messages on driving time inchoice of an itinerary.

4. A description of the way the VMS systemworks.

Figure 2 presents Stone’s formalization of the linksbetween the successive states entailed in the decisionto stay on a route or to abandon it when the VMSsystem announces a seemingly excessive drivingtime. The arborescent structure represents thealternative motives which compose the plan of action:the intention inferred from the reasons for choosingto go by automobile, to take the BP; knowledge aboutthe configuration of the system, the grounds on whichdrivers make their decision; the inferences they drawfrom the messages displayed on the VMS beforegetting on to the BP (action 1), then when they arealready on the BP and a traffic jam suddenly forms(action 2). In both cases, the decision is either tochange itineraries so as to avoid the jam, or to rest intraffic and think of something to do while waiting.

When there are traffic jams, what do drivers decide todo? According to results, we ascertained that thereare three different types of behavior regarding thisdecision : some (group 1, 7 participants, 23 %)change itineraries whenever traffic is slow, others(group 2, 18 participants, 60 %) stay in traffic nomatter what (even in the worst traffic jams), and stillothers (group 3, 5 participants, 16 %) will enter orstay in slower traffic providing that the driving timesdisplayed by the VMS’s are not excessive.

The first group (23 %) is composed of the driverswhose primary goal is to reach their destination asquickly as possible. These participants seem to befamiliar with alternative routes: 4 use secondaryroutes inside Paris (city streets). Because they knowalternative routes, they can quit the BP when traffic isslow. All of them use various sources of trafficinformation in addition to the VMS system beforedeciding on the route to take. All except one changeitineraries if when they arrive at the BP, the VMS’sindicate a traffic jam. If a traffic jam forms while theyare already on the BP, they get off. If the driving timethe VMS displays when they arrive at the BP seemsexcessive to them, they decide on an alternative routeand do not enter traffic on the BP. Their decision todesist is facilitated by the fact that in general theyhave a working knowledge of alternative routes,“emergency” routes to which they turn in case oftraffic jams. Thus, driving time information helpsthem avoid crowded routes.

In contrast to the first group are the group ofparticipants (60 %) on whom the system seems tohave no effect: they do not change itineraries whenthe driving time displayed suggests there are trafficjams. They seem to accept traffic jams very patientlyand are the most numerous among our participants.They never change their itinerary regardless of whatis announced on the VMS at the entry to the BP orwhile they are on it, preferring to make themselves ascomfortable as possible and wait out the time. Mostof them know of few alternative routes and do not tryto find out what these may be. Users in this categoryshow little readiness for finding alternatives to theBP, their knowledge of the network of roads outsideof the BP is limited, and this limits their ability tochoose an alternative route. However, even afteryears of driving, they do not seem to want to ventureoff their familiar itinerary, which they seem attachedto. Nonetheless, they also try to save time; some ofthem even leave earlier to avoid traffic jams. The

information on driving times reinforces theirconviction that “in any case, it’s the same on all theroads…”, and that there is little use in attempting tochange routes.

Finally, we ascertained that there was a thirdgroup of participants (16 %) who show behavioursomewhere in between the two behaviours describedabove. This category is composed of participants withmixed goals: some prefer the comfort of the ride,others emphasise speed. Unlike the previous group,drivers in this group do not get on the BP when atraffic jam had been announced. However, if a jamforms when they are already on it, they do not get off.They rely on the driving times displayed to maketheir decisions. All of them have a workingknowledge of alternative routes, and all of them alsouse the time spent in slower traffic to engage in otheractivities. It is very likely that the messages ondriving time reduce the stress of drivers in this group.As one of them says, If I see that the traffic jam isgoing to last for a while and that… oh well, too bad,I’m just going to have to wait it out. Like the secondgroup of drivers, reaching their destinations asquickly as possible is not always what most intereststhem, however, the reason they give for taking the BPis that it is faster, and in this they resemble the firstgroup. They will not get onto the BP if traffic isbacked up but nonetheless, like participants in thesecond group, they accept having to lose some time,though this acceptance is not unconditional andremains within limits that they can usually specify.They know the road network well and can changeitineraries when need be. They use VMS’s either toavoid getting onto a jammed BP (as indicated by aVMS or from what they can see) or, if a traffic jamforms when they are already on it, the informationhelps them to plan how to use the time spent waiting.

The assumption that drivers wish to reach theirdestinations as quickly as possible is not valid for all

drivers: only 23 % of participants emphasisedwanting to reach their destination as quickly aspossible and 60 % emphasised the comfort of driving.The whole set of results show that the implicit modelof the user which is a necessary component of theself-regulating system does not correspond to allusers. On the contrary, there are users who settledown comfortably into traffic jams: they maketelephone calls, listen to music, etc. As one of themstates very clearly :… I like the intimacy of my car…it’s like a small living room in which I can listen towhatever music I feel like hearing. I don’t have tostand contact with other people, I can read the paper,there are many things you can do in a car… Beingstuck in traffic is thus not necessarily something thatall drivers seek to avoid, since they know how longthey will be. It is not congestion that would maketrouble to the drivers, but to be keep in the dark aboutits duration. This “comfort” category of user does notcorrespond to the usual user model.

CONCLUSIONWhen a driver reads the driving time displayed on avariable message signpost (VMS) and the drivingtime indicates there are traffic jams ahead, twocourses of action are open to him: he can eitherabandon the road for another he believes will bequicker or stick to his itinerary. Our approach wasdesigned to reveal the modalities of this decision tostick to an itinerary or to change it, depending on theinformation delivered by the VMS’s (for example inParis, Porte d'Auteuil, 24 minutes), by studyinginterpretations of the contents of these messages interms of how the system functions (where the data isthought to come from), and the contextual processingof the displayed data (the driver’s task, how thetraffic looks) and finally decision making processes,but also the implicit model of the user for whom themessages are conceived and displayed.

REFERENCES

(1) Cohen, S., & Hadj-Salem, H. (1996)Évaluation comparative des affichages bouchonset temps de parcours. L’expérience de la ville deParis. Recherche, Transports, Sécurité, 51, 29-39.

(2) Jardin, P., & Laterrasse, J. (1998) Méthoded’analyse de l’impact des informationsdynamiques des panneaux à message variables surle comportement des usagers franciliens. In :Actes du Congrès International de l’ATEC« Déplacements : l’ère de la gestion. (pp 159-170). Paris : Presses des Ponts et Chaussées.(3) Tijus, C., Cambon de Lavalette, B., Poitrenaud,S. and Leproux, C., 2003, L’interactionautorégulatrice entre dispositif et utilisateur: unemodélisation des inférences sur les durées duparcours routier, Le Travail Humain 66:23-44.

(4) Tauber, M. (1988). On mental models and theuser interface. In G.C. Van Der Veer, T.R.G.Green, J.M. Hoc, & D.M. Murray (Eds.), InWorking with Computers: Theory versus Outcome(pp. 89-121). Londres: Academic Press.

(5) Poitrenaud, S. (1995). The Procope SemanticNetwork: an alternative to action grammars.International Journal of Human-ComputerStudies, 42, 31-69.

(6) Flanagan, J. C. (1954). The critical incidenttechnique. Psychological Bulletin, 51, 327-359.