Rumble strips are raised or grooved patterns on the roadway shoulder that provide both an audible warning (rumbling sound) and a physical vibration to alert drivers that they are leaving the driving lane. In addition to warning inattentive drivers, rumble strips help drivers stay on the road during inclement weather when visibility is poor.
The use of milled shoulder rumble strips (SRS) has been very effective in reducing single-vehicle run-of-the-road crashes caused by driver inattention, distraction, or drowsiness. Milled can be placed on either new or existing asphalt or cement concrete. A milled SRS is made with a machine that cuts a smooth groove in the roadway's shoulder. A SRS pattern results when SRS are repeated at regular intervals. This type of SRS modifies the pavement surface and provides for a vehicle's tires to drop, which creates high levels of vibrational and auditory stimuli. In Virginia, a 3-year (1997-2000) experiment with continuous shoulder rumble strips (CSRS) on the State's 1,476-kilometer interstate highway system showed that run-off-road crashes were reduced by 51.5 percent, saving an estimated 52 lives. Similarly, the judicious use of centreline rumble strips on undivided roads reduces the number of head-on collisions.
Mackie & Baas  refer to shoulder and centre line rumble strips as audio tactile profiled (ATP) edge and ‘no overtaking’ centrelines. Based on New Zealand data, Mackie & Baas  report favourable benefit-cost ratios (BCR) for ATP- treatments on roads with relatively modest traffic counts and much higher BCRs from higher traffic counts.