VTT Technical Research Centre of Finland Ltd has developed a system called the “Grip”, which, based on information collected from the vehicle data bus, is able to estimate road surface friction. The Grip system examines the difference between the rotational speeds of free-rolling wheels and driving wheels, and, when making calculations, takes the engine power into account. The result of this provides the slip ratio, which, in theory, increases in correlation to the degree of road surface slipperiness. This study compares the results obtained with the Grip method, using devices installed in tank tracks and buses, with friction measurement results obtained from a reference vehicle.
The measurements were performed by driving a private car equipped with friction meters behind the vehicles in which the Grip system was installed. The private car performing the reference measurements was equipped with three braking friction meters and one optical friction meter, RCM411. This continuous optical friction meter was the main reference meter.
In winter 2015-2016, four tests were performed using tank trucks and three tests using buses. Of the Grip data obtained during the tests, 87 % was recorded from the tank trucks, and only 38 % from the buses. A different type of vehicle unit was used in the buses than in the tank trucks, and the bus unit was not as reliable as expected.
The results from the five different test drives has presented in the figure below. The correlation between the slip ratio, determined using the Grip method, and the friction, measured with the optical friction meter, RCM411, was quite satisfying. The Grip method’s results were not as accurate as those of the RCM411 meter. However, the slip ratios calculated using the Grip method were almost invariably high when only lower friction levels measured with the optical meter. In road conditions with a lot of loose substances (snow, slush), the slip ratios calculated using the Grip method were lower than expected. Moreover, in slippery, more compact road conditions, the Grip system at times did not produce any slip ratio results at all. The reason for this can be found in the method itself, since the Grip system is only able to make observations when the vehicle is in driving mode. Since the primary function of the Grip system is to validate previous observations made by several vehicles, it can, based on the executed tests, be considered relatively promising.
Based on the executed tests, there is an increased quantity of slip ratio observations when the friction of the optical meter reaches the value of 0.45 or less. This corresponds to the friction value of 0.35 on the Finnish Transport Agency’s friction scale, which exceeds the friction requirement for the highest maintenance class (ls, which includes motorways) by 5 hundredths. Therefore, the Grip method could be implemented for roughly estimating the time when maintenance actions should be started (based on road slipperiness).
The accuracy of this method alone is not sufficient for assessing if the quality of the winter maintenance meets the requirements, but it might be sufficient enough for determining the service level of winter maintenance. According to the calculations made in this study, the Grip method might prove to be more cost-effective for this purpose than the system abandoned by the Finnish Transport Agency in 2011. In that system, consultants were hired to drive a vehicle equipped with friction meters along pre-determined, pre-scheduled routes. Based on this study, it is recommended that one or several pilot studies on the Grip method should be initiated. The pilot studies should include the end users of the Grip data (entrepreneurs, winter maintenance control, selected road users) and collect information about the end users’ views on the method.
The report can be found in the Finnish Traffic Agency website (there is a large 16 page summary in English in the end of the report. The report is here.