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Journal Article


Malmivuo M. Res. Rep. Finn. Transp. Agency 2011; (14): online.


(Copyright © 2011, Finnish Transport Agency)






Several different surveys show that the risk of traffic accidents is considerably higher under winter road conditions than summer road conditions. The aim is to reduce this risk by means of winter maintenance. In Finland, Sweden and Norway the winter maintenance policy is that roads do not have to be bare in winter, as long as there is sufficient friction on the road surface in accordance with the quality requirements specified for winter maintenance. By enforcing this principle, excessive use of road salt can be restricted. Since the choice of friction measurement method affects the measured friction values, the above-mentioned countries have specified the devices by means of which the friction referred to in the winter maintenance quality requirements has been measured. This survey aims at charting the experience of the various new friction measuring devices in different countries through literary research and interviews. In addition, the purpose of the survey is to give real-time background information in support of friction measurement tests in Finland. In Finland the measuring of road surface friction has for more than 20 years largely been based on a system, in which a vehicle-integrated additional device measures the deceleration during braking on the basis of the change in the vehicle wheel rotation speed. However, in the last few years recently developed devices for measuring spot and continuous friction have entered the market. In Finland it is mainly friction measuring devices based on braking friction and so called optical friction sensors that have been tested in the 21st century. Continuous friction measurement has primarily been introduced through a device which measures side friction, developed by a Finnish entrepreneur. This device has been installed in buses and tested in American-type vans. However, so far the experiences of continuous friction measurement have not been very encouraging. In Sweden various continuous friction measuring devices have been tested in the last few years, although these devices still have not achieved official status in winter maintenance management. At the time of writing, it seems at this point as if the Swedes are more interested in the devices by an American manufacturer (product names RT3 and RT3 Curve). The testing of the devices by the manufacturer in question continues in Sweden during the winter 2010-2011. In these measuring devices the measuring wheel is an ordinary car tire. The advantage of this is that the measuring wheel is likely to behave in a manner which is as similar as possible to the car tire. In addition, it is considerably cheaper to replace a measuring tire than a specially manufactured measuring tire. At the moment the continuous friction measuring devices are mostly used and tested in Norway, where they have already been given the status of official measuring device in some areas with contracts for winter maintenance. In Norway a policy has been adopted, according to which a few expensive continuous measuring devices, which are as accurate as possible, have been acquired to be used as reference measuring devices. In addition to these, tens of cheaper continuous measuring devices have been acquired, which are regularly calibrated against these reference devices. In North America friction measuring devices do not play an official role in winter maintenance, even though they are used quite extensively in different surveys. In North America measuring devices based on braking friction are avoided due to their implied traffic safety problems. Friction measuring devices have been tested in various countries both in a road environment and under more controlled circumstances on airports or ice-covered lakes. The road environment is challenging due to its non-homogenous conditions. The conditions on the test tracks are more homogenous, but on the other hand quite artificial in comparison to the actual road conditions. A research plan, based on the results of this preliminary study, has been drawn up for testing new friction measuring devices equipped with acceleration sensors and for comparing these with the traditional braking friction measuring devices and the mobile version of the so-called optical sensor. In this survey the devices will be tested both under road conditions and on a test track. The research plan has been described in a separate document. As a result of this survey, it is recommended that a separate, extensive cost-benefit analysis will be carried out for the more expensive continuous friction measuring device. This report may be found at


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