The Finnish National Road Administration (FinnRA) launched the experiment of variable road condition warning signs during the winter of 19931994. The results showed that the sign warning about slippery road conditions reduced the mean speeds. In addition, the variable safety margin sign extended the headways and reduced mean speeds.
The experiment was continued during the winter of 19941995. The signs were tested at four sites: Eurajoki (vt 8), Koski (vt 10), Salo (vt 1) and Koikkala (vt 1). This study was designed to investigate the effects of the variable signs on vehicle speeds and headways, and the persistence and duration of the effect. In addition, the understanding of the sign was studied, and user experiences were noted.
The signs used in the experiment were variable road condition warning signs equipped with an additional snowflake sign or without a snowflake, and a variable safety margin sign of an individual safety margin recommendation between the illustration of two cars. In the middle of the winter, the word "recommendation" was added underneath the sign. Information about driver behaviour was collected using detector loops and an instrumented car. In addition, roadside interviews were made as well as a questionnaire to the personnel of the road condition centre.
The effects of the variable message sign warning about slippery road conditions remained about the same as in previous winter at Eurajoki and Koski. The speed-reducing effect of the sign was about 2 km/h when the sign was blinking, and about 1 km/h when the sign was on continuously. In Salo, the road condition sign did not reduce speeds but the sign decreased the portion of headways less than 1.5 seconds by 10 percent. At the Koikkala curve, the sign had a statistically significant effect on the driving speeds both in the vicinity of the sign and on the straight section 1200 meters from the sign. The duration of the speed effect of the sign warning about slippery road conditions was about 3 kilometres. The blinking sign affected at least at the distance of 14 km.
The variable safety margin sign decreased the proportion of the drivers in queues with headways of less than 1.5 seconds by 2838% during good road conditions and by 3137% during slippery road conditions. The effect was decreased after the addition of the word "recommendation". In addition, the sign decreased the speeds by about 1 km/h. The purpose of the slippery road warning sign was correctly understood by 80% of the drivers (n=213). The proportion of drivers who understood the variable safety margin sign correctly was 84%, after the addition of the word "recommendation". 4550% of the drivers didn't know the meaning of the different ways to use the sign (blinking/continuous).
Both of the signs changed driver behaviour into a safer
direction. The slippery road warning sign can be used at locations that are
especially prone to slipperiness. The operation of the sign requires that the
road conditions are under continuous surveillance. The additional snowflake sign
is not necessarily required. The sign should probably only have two modes of
operation: it is either lit, warning of slipperiness, or off. The use of the
blinking option remains to be considered taking into account the whole traffic
control system and the information management processes of drivers. The safety
margin sign affected as such both mean speeds and headways. Hence, the sign
displaying a safety margin recommendation on the basis of prevailing road
surface condition and vehicle speed should be developed further. It is still
unclear whether the effects of the signs would increase or decrease if they were
used more extensively.