Company Logo Add Your Company Slogan Pirkko Rama *, Risto Kulmala Transportation Research Part F 3 (2000) 85±94 學生. 莊靖玟 Effects of variable message signs for slippery road conditions on driving speed and headways

Company Logo Purpose  This study investigated the effects of two variable message signs (VMS) on driver behaviour.  This study was limited to investigating how VMS for slippery road conditions affect driver behaviour in terms of speed and headways.

Company Logo Reference 年份學者結果 1997Malmivuo & Peltola The risk of injury accidents in winter has been estimated to be over 20 times higher on an icy road surface than on a dry and bare one. 1994Heinijoki On slippery road surfaces, only 14% of Finnish drivers estimate the road to be slippery, while over half regard the friction as quite normal. 1993Saastamoinen Average speeds on a slippery road surface are roughly 4 km/h lower than in good winter conditions, and the standard deviation of speed is also lower due especially to a drop in the highest speeds.

Company Logo Reference 年份學者結果 Roine ; Malmivuo & Peltola This adaptation has been proved insufficient to fully compensate for the reduced friction 1993Saastamoinen The percentage of headways shorter than 1.5 s was found to be approximately 25% in slippery conditions compared to 39% in good winter conditions.

Company Logo Method Signs  Two VMSs were evaluated: (a) a slippery road condition sign (b) a sign recommending the minimum headway between vehicles

Company Logo Method Signs  The TMC operators classified road surface conditions in three categories for the purpose of VMS control: (a) good (b) possibly slippery (c) verified slippery  The slippery road condition sign is on or off and the minimum headway sign is always on.

Company Logo Method Design and sites  There were three sites in southwest Finland (Site 1 at Eurajoki, Site 2 at Koski and Site 3 in Salo).  The posted speed limit at the sites was 80 km/h.  The set-up shown in Fig. 2 is for Site 1 but was the same at Sites 2 and 3, with the exception that there was no minimum headway sign.  The study included data from two winter periods at Sites 1 and 2, and from one winter period at Site 3.

Company Logo Method Design and sites Fig. 2. Experimental set-up (TS. Traffc monitoring Station).

Company Logo Method Design and sites  The experimental design included six experimental sign conditions as follows: (1) steady slippery road condition sign (2) flashing slippery road condition sign (3) minimum headway sign + steady slippery road condition sign (4) minimum headway sign + flashing slippery road condition sign (5) minimum headway sign only (6) nothing (black sign)

Company Logo Method Design and sites  Speed analysis. ＊ Only cars travelling in free traffic flow conditions (with a minimum headway of 5 s to the vehicle ahead) were included. ＊ The following formula was used to compute the effect of the VMS:

Company Logo Method Design and sites  Headways. ＊ We only included vehicles with a headway of 5 s or less in the analysis. ＊ To estimate the effects of various factors we used a log-linear model to describe the proportion of short headways as follows:

Company Logo Results Speed  We assumed the effect of the VMS on mean speed to be dependent on the speed level before the VMS.  In good road conditions, the mean speeds at TS1 were 87 km/h (Site 1), 91 km/h (Site 2) and 84 km/h (Site 3).  In adverse road conditions, the mean speeds were 5 km/h lower on average.

Company Logo Results Speed

Company Logo Results Speed  First, at the monitoring station(TS3) ＊ The flashing slippery road condition sign decreased the mean speed on average by 2 km/h (P<.05). ＊ The steady display at TS3 had no significant effects at Site 2 and significant increasing effects (on average 0.9 km/h, P<.05) at Site 1.  Second, the mean speed effect was checked for light conditions (i.e. day versus night).  Third, the effects on the speed distribution were examined.

Company Logo Results Headways

Company Logo Discussion  (Ranta & Kallberg, 1996) ＊ Decreases in average speed are related to reduced accident risks.  ( Rama et al.,1996) ＊ The majority of drivers regarded both signs useful: 65% approved of the slippery road condition sign and 72% the minimum headway sign. ＊ Seventy percent of drivers in the first winter and 80% in the second winter interpreted the meaning of the slippery road condition sign correctly.

Company Logo Conclusions  The results showed that the slippery road condition sign reduced the mean speed on slippery roads by 1-2 km/h in addition to the decrease caused by the adverse road conditions.  The minimum headway sign decreased the proportion of headways shorter than 1.5 s for cars in car-following situations, in addition to a speed reduction of 1 km/h.  The effects were somewhat smaller during the second winter than the first.