As traffic flow levels increase, there is a growing demand for mobile measurement methods. On busy roads, it is not safe to use hand-held instruments as measurement staff must work on the road. Instead, both for the safety of staff and drivers, mobile measurements in traffic speed are preferred. This study presents an evaluation of a mobile reflectometer, the LTL-M, developed by DELTA Light & Optics in Denmark. In practice, a vehicle based retroreflectometer will have some tilt or lift relative to the road marking surface caused by movements of the vehicle or camber of the road. This leads to changes of the actual distance, either randomly or systematically, and thereby changes of the measured RL value because of the distance law of illumination. The LTL-M uses an optical system, which may be described as a simulation of the defocused system, refer to section 2. This optical system should make measurement reliable, meaning only small systematic and random errors. In the first phase of the study, the two optical principles (focused and defocused) were tested and compared in the laboratory by varying length, height and measurement angles. Thereafter, a field study, involving measurements of several types of road markings in Denmark and Sweden was carried out. In this second phase, the two mobile instruments, the LTL-M and Ecodyn 30, were used and the reference instrument was the hand-held LTL-2000. Furthermore, repeatability was studied by measuring each test section twice. The result of the laboratory study showed that readings using the focused optical system (Ecodyn 30) may suffer from large measurement errors due to changes in measurement geometry. Contrary, the readings of the LTL-M, using the defocused system, were almost independent of the measurement geometry. Consequently, the laboratory measurements clearly indicated that the defocused optical system is preferable on a mobile reflectometer. In the field study, the readings of the LTL-2000 were seen as the key, although even these measurements suffer from errors. Furthermore, the readings are not entirely comparable as the LTL-2000 was used to measure only a small percentage of the road marking area, while the LTL-M measured virtually all of the area. The conclusion from the laboratory and field measurements is that the LTL-M measures with less systematic and random errors compared to the Ecodyn 30. Furthermore, the repeatability of the LTL-M is better. Finally, it is suggested that a larger field study is carried out. This study would include not only the LTL-M and Ecodyn 30, but also other mobile instruments on the market. (Author/publisher)
Abstract