Overview
For road markings, there is a performance requirement regarding friction, which is given in units obtained with handheld instruments. Such instruments are not suitable for assessment of friction on longer stretches of road, since it would be too time consuming and expensive.
In order to be able to monitor a large road network and to assess whether the performance requirements are fulfilled, mobile measurement methods are needed, i.e. methods based on instruments mounted on vehicles. Besides, the friction that occurs between the tyres of a vehicle and the road surface is speed dependent. Handheld instruments reflect the friction that occurs in low speeds, while from a traffic safety perspective, friction in higher speeds is most relevant.
The present report describes the results from two studies about methods for mobile assessment or estimation of road marking friction. In the first study, three different mobile methods, Road Friction Tester (RFT), Traction Watcher One (TWO) and Road Marking Tester (RMT), were compared to the handheld method Portable Friction Tester (PFT). In the second study, a prediction model for friction in higher speeds was developed, based on measurements of the coefficient of retro reflected luminance and the texture of the road marking.
Results
The first study showed that the different methods give different results, which is expected since the physical quantity coefficient of friction does not have a unique definition but is dependent on the properties of the instrument. In lower speeds, there were significant relationships between PFT and RFT, and between PFT and TWO (correlation coefficient 0,89 and 0,86, respectively).
The relationships between PFT and RMT were somewhat weaker, although significant (correlation coefficients 0,65–0,71). In higher speeds, there was a significant relationship between RFT and TWO (correlation coefficient 0,80). The relationships between RMT and RFT, and between RMT and TWO, were weaker but significant (correlation coefficients 0,86–0,89). TWO gave lower friction values than the other methods. Measurement speed had an influence on the results obtained with RFT and TWO. In the second study, TWO was used as a reference when developing a prediction model for friction in higher speeds. The results showed that friction to some extent can be predicted from the coefficient of retro reflected luminance and the texture of the marking, but that the relationship is not strong enough for the model to be applicable.
A limitation of the study is that there is no standardized method for measurement of road marking friction in higher speeds available. The reference method used, TWO, is developed for friction assessment of road surfaces and not for road markings, which implies that the ability of the instrument to assess road marking friction is somewhat uncertain.