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Pirkko Rama *, Risto KulmalaTransportation Research Part F 3 (2000) 85±94

學生 .莊靖玟

Effects of variable message signs for slippery road conditionson driving speed and headways

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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.

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Reference

年份 學者 結果1997 Malmivuo & Pel

tola 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.

1994 Heinijoki On slippery road surfaces, only 14% of Finnish drivers estimate the road to be slippery, while over half regard the friction as quite normal.

1993 Saastamoinen 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.

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Reference

年份 學者 結果19931997

Roine ; Malmivuo & Peltola

This adaptation has been proved insufficient to fully compensate for the reduced friction

1993 Saastamoinen 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.

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MethodSigns

Two VMSs were evaluated: (a) a slippery road condition sign (b) a sign recommending the minimum headway between vehicles

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MethodSigns

The TMC operators classified road surface conditions in three categories for the purpose of VMS control:

(a) good (b) possibly slippery (c) verified slipperyThe slippery road condition sign is on or off

and the minimum headway sign is always on.

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MethodDesign 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 sa

me 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.

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MethodDesign and sites

Fig. 2. Experimental set-up (TS . Traffc monitoring Station).

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MethodDesign 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)

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MethodDesign 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:

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MethodDesign 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:

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ResultsSpeed

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.

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ResultsSpeed

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ResultsSpeed

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 cond

itions (i.e. day versus night). Third, the effects on the speed distribution were examine

d.

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ResultsHeadways

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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.

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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.