Eyes on the road
The SCRIM® trucks were back on New Zealand’s highways over the summer of 2015-16, completing their surveying over the North and South Island regions by January 2016.
SCRIM is the acronym for Sideway-force Coefficient Routine Investigation Machine. The distinctive yellow tucks are commissioned by the New Zealand Transport Agency and have become a feature on the Country’s roads as they travel the networks at 80 kilometres per hour collecting pavement information – skid resistance, texture, roughness, rutting, geometry (gradient, cross-fall and curvature) and centreline GPS coordinates.
In addition to responding to found defects, NZTA uses the data to monitor highway performance; plan future work programmes; analyse trends; and predict how road conditions might change in the future. Information is stored in the agency’s RAMM database and used for many aspects of network modelling.
The two SCRIM vehicles are unique to New Zealand, designed to deliver data as required by NZTA. They are built and supplied by Bristol-based W.D.M. Limited, a specialist manufacturer and provider of highway surveying and monitoring equipment and the largest survey contractor in the UK, serving all UK government agencies and 90 per cent of local authorities.
The Company also carries out work in South Africa and Australia, among other overseas markets, and has been surveying New Zealand’s road network every year since 1995, with a focus on the Transport Agency and local authorities.
The surveying is very accurate. The trucks return to the UK each year for a complete overhaul of the technical equipment and every nut and bolt on the vehicles.
Collected data also goes through multiple validation checks for accuracy. John Donbavand, National Pavements manager at NZTA, calls it “the largest validation programme in the world”, with the machines put through rigid ‘repeatability’ tests using comparative data.
The GPS systems on the SCRIM means the Transport Agency can match 20 metre segments of road between different years, he says.
During 2014-15, John adds, the NZTA had two SCRIM trucks take part in repeat tests on the same section of road in Central Otago to compare data for accuracy, and with satisfying results.
While the truck is on the move, a bar with 20 lasers defines the transverse profile over a road width of 3.3 metres. The rut depth in each wheel path is calculated from this transverse profile using a simulated two metre straight edge. The data is used to determine the average, maximum and minimum rut depth and the standard deviation and the distribution of rut depths every 20 metres.
Longitudinal profile (roughness) is measured using two lasers, one in each wheel path, together with accelerometers, and deducted from the laser output to provide a road profile. The International Roughness Indicator (IRI) is calculated from the longitudinal profile using the World Bank Quaeter Car model and, again, is reported every 20 metres.
Road texture is measured by three 32 kilohertz lasers, one each in the left and right wheel paths and one between the wheel paths. Accelerometers remove most of the vehicle motion relative to the road to provide a stable inertial profile from which the MPD (Mean Profile Depth) is calculated. The MPD is also measured and calculated according to ISO 13473-1:1997.
Survey equipment features a geometry measurement system using inclinometers and gyroscopes to sense the vehicle attitude as the SCRIM travels along the road. Inclinometers measure the forward or back tilt of the vehicle for gradient and the side-to-side tilt of the axles for cross-fall.
Gyroscopes are used to sense the movement of the vehicle around corners to calculate the horizontal and vertical curvature. Geometry data is smoothed with a 30-metre moving average and reported at every 10 metres.
Oxford Technical Solutions GPS equipment samples the Omni-Star satellite to record the differential GPS coordinates of the centre-line. Tilt sensors for cross fall and gradient, together with a gyroscope, provide alignment details when out of sight of the satellite. Information is post processed using LINZ base stations.
The survey data, including forward facing video, is then sent back to WDM®’s offices in Wellington where it is processed, fitted to the NZ road network and verified before being issued to NZTA. High data integrity standards are applied by WDM® to ensure clients can rely on SCRIM measurements in making investment decisions.
Skid resistance is a vital feature of road safety in a country such as New Zealand, with its abundant rainfall, ice, snow and surface water during heavy rain, which is thrown into the face of oncoming traffic by trucks.
WDM® promotes its work on skid resistance in NZ on its website. “The wet skid injury crash rate on road sections with ‘low’ skid resistance is 4.5 to nine times greater than that for all roads,” the Company says.
“Results of crash site analysis indicate that a 0.1 increase in skid resistance (measured in terms of Mean Summer SCRIM coefficient) causes a reduction in injury crashes of 30 per cent on wet roads and 20 per cent on dry roads for the NZ state highway network. Overall, since SCRIM surveys were introduced, the number of skid-related fatalities in New Zealand has fallen by nearly 40%.”
To obtain skid resistance information, the SCRIM features a freely rotating test wheel that is applied to the road surface under a known load. A controlled flow of water wets the road surface immediately in front of the wheel, so that when the vehicle moves forward the test wheel slides in a forward direction on the wet road surface. The force generated by the resistance to sliding indicates the wet road skid resistance of the surface. The results of this testing are averages to determine the skid resistance on continuous 10 metre sections of the road.
The problem has been that while SCRIM data can identify which aggregates are proving best for skid resistance, contractor records of where the quarry aggregate actually comes from are sometimes found lacking. As a result, NZTA has not been getting the pavement performance it has expected from some road projects and says it is having ongoing ‘healthy conversations’ with the industry as to why.
* SCRIM is the registered trade mark of W.D.M. Limited