On the weather-controlled E18 road in Southeast Finland speed limits are controlled by data from unmanned Road Weather Stations (RWSs). The speed limits are lowered automatically during adverse road conditions and in some cases signs for slippery road conditions are displayed as well. The total length of the weather-controlled road is currently 25 km. Six RWSs along this section collect standard meteorological data, which is fed automatically every 5 minutes to the central station for storage and analysis. The central unit of the road weather information system also analyses road conditions and recommends speed limits accordingly, transmitting them to the traffic signs' control logistics equipment. All signs have both data and electric wiring and are connected to the technical building by a standard industrial bus. The signs can also be controlled manually. The effects of the system on the two-lane section were as expected if the system is appropriately used. The results showed that raising the speed limit from 80 km/h to 100 km/h increases the mean speed by 3.9 km/h for cars travelling in free flow traffic in good road surface conditions. Under adverse road conditions the VMSs decreased the mean speed and increased the headways between vehicles, which is desirable for traffic safety. First, lowering the speed limit from 100 km/h to 80 km/h decreased the mean speed of cars travelling in free flow traffic by 3 km/h during poor weather conditions. Second, if the slippery road warning was displayed the mean speed decreased by 2.5 km/h in poor road conditions. Third, the slippery road sign improved traffic safety by reducing the proportion of headways less than 1 second in queues 80 km/h but displayed with fibre-optic signs (instead of fixed signs), the mean speed decreased in good road conditions by 3.2 km/h but increased in poor and moderate road surface conditions by 1 km/h. The result indicates the effectiveness of fibre-optic variable speed limit signs. They are experienced by drivers not only as the highest allowed speeds, but also apparently as recommendations. The same phenomenon is obvious in the inappropriate use of the variable speed limit system when the highest speed limit was shown contrary to the control policy. The increase of mean speed seemed to be dependent on the weather and road conditions and was very substantial (7.3 km/h) in poor conditions. If the 100 km/h speed limit was shown during normal conditions against the control policy, the proportion of short headways increased substantially. It would seem that the overly high speed limit prevented drivers from taking into account the adverse road conditions. Drivers are accepting VMSs on the E18 road. Most of the drivers (96 per cent) considered variable signs based on real-time weather and road condition data to be worthwhile. VMSs were also recalled well compared to fixed signs. Interviews have shown that drivers are largely aware tha weather conditions determine the posted speed limits (Raemae and Luoma 1997), but when drivers were asked in greater detail about this only 56 per cent of them thought that control was based on sensors (collecting real-time data). This lack of understanding of control strategies implies that there might be some potential for increasing the effects of the system. The drivers assessed the system to improve traffic safety, fluency of traffic flow and compliance with speed limits. Based on observations and friction measurements, the speed limit and use of the sign for slippery road were estimated to be in agreement with the control strategy and circumstances in 76 per cent of cases. There is a need for further development of an error-free system. Meanwhile, manual control is still necessary to aid the system in identifying slippery conditions. The findings suggest that the use of variable speed limits calls for a sophisticated control system. Inadequate speed limits increase the mean speed excessively and decrease headways substantially, compromising traffic safety. Drivers both accept variable speed limits and rely on the system. (A) For the covering abstract of the conference see ITRD no 207828. The reprints are also available at the web - http://www.vti.se/pdf/reports/K18APart1.pdf; http://www.vti.se/pdf/reports/K18APart2.pdf and http://www.vti.se/pdf/reports/K18APart3.pdf.
Abstract