3 Speed Management
3.2.6 E18 Variable Speed Limit System – Finland
Impetus for the Activity
Traffic safety, traffic fluency, and improving drivers’ motivation to obey the speed limits were the impetus for developing the system.
In Finland, “winter speed limits” are used. Normal speed limits are lowered 20 km/h during winter on certain roads. However, the problem is that for most of the winter, there are summer road conditions in the southern part of Finland, and the drivers have difficulties being motivated to obey the speed limits. This lack of motivation to obey the speed limits also causes frustration with the other traffic rules. When variable speed limits are used, it allows low limits during bad and slippery road conditions and higher limits during good road conditions, thus motivating the drivers to obey the speed limits and the other traffic rules.
Criteria for System Selection (Hardware and Software)
The system was the first of its kind. It was planned by FINNRA (Finnish National Road Administration) and constructed with selected manufacturers and software companies. In 1992 when FINNRA started planning the system, there was no system available that would automatically change the speed limit and information signs in response to the road conditions.
Description of the System
The Finnish system is installed and active. The first 12-km section was tested in 1994 and has been in use since then.
A second 13-km section was added in 1997 to make a total length of 25 km.
The objective of the system is to influence driving behavior and improve road safety without decreasing driver motivation to obey posted speed limits.
The system is deployed in a rural environment on E18 in Southern Finland between Kotka and Hammina.
There is an additional system deployed on a 50-km road section, also on E18, between Lohjanharju and Salo (1998) and some additional systems were installed on other road sections in 1999.
The system has 67 VSL signs and 13 VMS signs over the 25 km of E18. The roadway is a motorway and dual carriage road. The system has five unmanned stations to monitor local weather (wind velocity and direction, air temp., relative humidity, rain intensity, and cumulative precipitation) and road surface conditions (dry, wet, salted, snowy - through four sensors per station). These measures are used as inputs to the system.
Inputs are sampled once per minute in the station and collected every 5 min. Wind information is collected as 10-min averages and gusts. Instantaneous values of temperature are used. The road conditions are averaged over the sampling period, and trends are considered. Instantaneous rain measurements are used and the accumulation is also tracked and used.
The inputs are analyzed by a central unit that gives the speed limit recommendation:
- 120 km/h for good road conditions,
- 100 km/h for moderate road conditions, and,
- 80 km/h for poor road conditions.
The system output is the posted speed limit and a slippery road warning may also be given (as determined by the central unit). This information is posted on VMSs, but Finnra is also testing the use of the Internet and communications devices so that the information could reach the drivers with in-vehicle devices.
The output is updated when the road condition classification changes, for instance from moderate to bad. When the conditions are getting worse, the output is updated immediately. When the road conditions are improving, the output is updated after the improved conditions have prevailed for 15 min.
The speed limit posted by the system is enforced. A picture of one of the system’s VSL signs is shown in Figure 3.
A number of factors affect system performance. These include:
FINNRA feels that for the first system of its kind, it has performed satisfactorily, but they have performed many improvements and some additional development is still to be done. |
 Figure 3: A VSL sign on E18 in Finland. |
System Effectiveness Evaluation
The effectiveness of the Finnish VSL system has been formally evaluated. The evaluation was performed by an independent party, VTT (Technical Research Center of Finland). The evaluation included:
- A before and after study,
- The use of a control road,
- User interviews, and
- Video camera studies concerning bilingual information signs.
Measures of effectiveness included:
- Effects on traffic speeds (mean values during different driving conditions),
- Effects on speed variation,
- Effects on headways, and
- Effects on traffic fluency.
The evaluation showed that the system achieved the expected results, namely:
- It had an effect on the mean speeds and the deviation of speeds during bad driving conditions;
- It was well accepted and it seemed to increase the drivers’ motivation to obey the VSL better than “normal winter speed limits”;
- 95 percent of drivers interviewed endorsed the use of speed limits set according to the prevailing road conditions;
- The concept of weather-controlled speed limits and displays is promising.
Perceived “Hot Spots” for Rural ITS Deployment
Perceived “Hot Spots” identified on the questionnaire included:
- Areas where road conditions on main roads vary from summery conditions to very difficult wintry conditions throughout the winter; and
- Foggy areas where the wind or heavy rain can cause problems for the traffic during summer.
Obstacles and Lessons Learned
The reliability of the weather and road surface condition data is very important. In addition, the upkeep of the system is of utmost importance. Problems were experienced with thunderstorms, damage of the signs, and data transfer devices.