Safe, reliable highways start with knowing their condition. That’s why we regularly check how well our roads perform—looking at grip, smoothness, and overall shape. Using a specialised vehicle called SCRIM+ (Sideway-force Coefficient Routine Investigation Machine), we gather detailed information about the road surface while driving at normal speeds. This data helps us spot issues early, plan maintenance efficiently, and keep everyone moving safely without unnecessary delays or costs.
SCRIM+ survey vehicle (image courtesy of WDM Ltd).
Each year, from October to March, we carry out SCRIM+ surveys to check key aspects of road safety. We collect information on:
The survey vehicle also records a 'network video' – what you see as you drive the highway.
Along with responding to immediate condition, we also use the results to:
SCRIM+ collects a wide range of data to assess road safety and performance. Some of the terms used in these measurements can be technical, so here’s a quick guide to what they mean and how they help us understand the condition of the road.
All measurements are taken in one go while the SCRIM+ vehicle drives at up to 80 km/h.
A bar with 20 lasers measures the road’s cross-section over a width of 3.3 metres. From this, we calculate how deep the wheel tracks (ruts) are using a simulated straight edge. We then work out the average, highest, and lowest rut depths, plus how they vary, every 20 metres.
Two lasers and sensors measure how smooth or rough the road is along its length. The sensors remove the effect of the vehicle’s suspension, so we get an accurate road profile. From this, we calculate the International Roughness Index (IRI), which shows how bumpy the road is, every 20 metres.
Three high-frequency lasers measure the fine surface texture of the road in both wheel tracks and the middle. Sensors correct for vehicle movement so we can calculate the Mean Profile Depth (MPD), which shows how coarse or smooth the surface is, following international standards.
Sensors measure the slope and curves of the road. Inclinometers detect uphill/downhill and side-to-side tilt, while gyroscopes measure bends and curves. This data is smoothed and reported every 10 metres.
The SCRIM+ uses GPS (Global Positioning System) to record its exact position on the road. If the satellite signal drops, tilt sensors and gyroscopes keep track of alignment.
A test wheel slides on a wet road surface under controlled conditions. The resistance to sliding shows how much grip the road has when wet. Results are averaged for every 10-metre section.
