Also known as audio tactile profiled (ATP) road markings, rumble strips are an effective way of letting drivers know when they are drifting out of their lane or onto the shoulder, by creating noise and vibration inside the vehicle.
Every year the NZ Transport Agency (NZTA) spends around $4 million installing ATP road markings on roads around the country. Research shows that the markings reduce road accidents, but a major issue for the NZTA was determining when to replace them. WSP Research developed a study, funded by NZTA, to measure the effects of ATP design and wear-and-tear on the noise and vibration levels produced, and the threshold at which drivers no longer perceive them.
Plastic or wooden blocks of a range of heights and at variable distances were used to simulate new ATP markings.
A test car was driven over each strip at differing speeds and the noise and vibration levels inside the vehicle were measured each time. The effects of the different blocks and their position, on both the overall noise levels and the quality of this noise were analysed by our noise modelling team.
Our Behavioural Scientists then examined driver response to the changes in noise and vibration from the different block heights to determine the height at which drivers would reliably detect ATP markings, using a specially designed simulator. Drivers had to reliably identify the markings as distinct from other sounds, such as road noise and seal joins, while performing other tasks that replicate the demands on a driver’s attention, and both with and without background music.
Using signal detection theory, a method used in psychology for determining sensitivity to stimuli, a height of 4mm was found to be the threshold at which drivers reliably perceive the ATP markings. However, as driver age and the presence of background music were found to reduce the driver’s ability to detect the markings, a more conservative 5mm height was recommended. Through this research, WSP Research was able to provide NZTA with a basis for future maintenance plans to ensure ATP markings remain effective in warning drivers, while reducing costs by not replacing markers until this is necessary.
View the full research report on the NZTA website