Testing the Skid Resistance of Asphalt Surfaces for Safer Roads

A road can look smooth, recently sealed and structurally sound while still providing inadequate grip in wet conditions. Skid resistance is the ability of an asphalt surface to generate friction between a vehicle tyre and the pavement. It affects braking distance, cornering control, motorcycle stability, pedestrian safety and the likelihood of losing control at intersections, curves and approach zones.

For Australian road authorities, contractors and asset owners, measuring pavement friction is an important part of quality assurance and network maintenance. Rainfall, intense summer heat, heavy traffic, rubber deposits, aggregate polishing and surface contamination can all change how a road performs over time. Reliable field testing helps turn these risks into measurable maintenance decisions.

Why Asphalt Grip Changes Over Time

Skid resistance depends on two related surface characteristics: microtexture and macrotexture. Microtexture is the fine roughness of individual aggregate particles. It provides the small-scale grip needed when a tyre is braking or travelling through a wet film. Macrotexture is the larger pattern formed by the spaces and surface profile between aggregate particles. It helps water escape from beneath the tyre, reducing the risk of aquaplaning.

A pavement may have adequate macrotexture but poor microtexture if traffic has polished the aggregate. This is common in heavily trafficked lanes, bus corridors, freight routes and approaches to signalised intersections. Conversely, a coarse surface with strong texture can still perform poorly if it is contaminated by diesel, oil, loose material or excessive rubber build-up.

Temperature and weather also influence results. In a city such as Melbourne, a polished intersection may receive repeated rainfall and braking loads across the year. In Brisbane or Sydney, intense storms can place sudden demands on surface drainage and wet-weather friction. In Perth, dust and dry climatic conditions can affect contamination levels, while high summer pavement temperatures may accelerate binder ageing and aggregate wear.

The condition of the whole pavement system matters as well. Rutting can hold water in wheel paths, and depressions can create localised wet areas. Drainage defects, edge failures and uneven transitions may increase crash risk even when a single friction reading appears acceptable. For that reason, skid testing should be considered alongside texture, rut depth, drainage, pavement distress and traffic exposure.

How Field Testing Measures Friction

Several test methods are used to assess the frictional performance of asphalt. A locked-wheel skid tester measures the braking force generated when a test tyre is prevented from rotating while travelling over a wetted surface. The resulting friction value is useful for evaluating wet pavement behaviour at a defined test speed and under controlled conditions.

The British Pendulum Tester provides a portable approach for measuring low-speed slip resistance. It is particularly useful for local investigations, small areas, pedestrian surfaces, bridge decks, ramps and locations where a full-scale continuous friction machine is impractical. Results can be influenced by surface texture, test direction, temperature, water application and the condition of the slider.

Continuous friction measurement equipment can test longer road sections at traffic-relevant speeds. These systems apply controlled water to the pavement and record friction continuously, producing a profile that reveals short sections with unusually low grip. Such data is valuable for network screening, site acceptance, black spot investigations and before-and-after treatment assessments.

Surface texture may be measured separately using methods such as volumetric sand patch testing, laser-based profiling or other approved texture instruments. The result helps explain a friction reading. A low texture depth may indicate flushing, binder-rich surfacing or polishing, while a high texture depth does not automatically confirm adequate wet-weather grip. Test selection should reflect the road type, investigation purpose, safety requirements and applicable Australian specifications.

Soil and pavement support conditions should not be ignored when interpreting surface performance. Weak subgrade, moisture movement or poor drainage can contribute to rutting and deformation that retain water in vehicle paths. Where a broader pavement investigation is required, soil testing services can help establish whether underlying conditions are contributing to surface-related safety concerns.

Interpreting Results With Engineering Context

A friction number should never be treated as an isolated pass-or-fail statement without considering the test method and site conditions. Readings are affected by speed, tyre type, water depth, test direction, ambient temperature, pavement temperature and the cleanliness of the test area. A result from a portable low-speed instrument cannot be compared directly with a continuous high-speed friction survey unless the methods have been correlated.

Engineers generally assess both average performance and localised minimum values. A long road section may have an acceptable mean friction level but still contain a short, hazardous zone at a stop line, curve or crest. Mapping readings against chainage, lane position and road geometry makes it easier to identify patterns and prioritise corrective work.

Repeatability and traceability are central to a credible assessment. Equipment should be calibrated, operated by competent personnel and checked before and after testing. The report should record weather, surface condition, test speed, water application, tyre or slider details, test direction, location coordinates and any unusual contamination. Photographs and texture measurements can provide useful evidence when a result needs to be reviewed later.

Testing laboratories and field teams that work under a quality system can provide a stronger basis for project decisions. An accredited approach aligned with SNI ISO/IEC 17025 supports technical competence, documented procedures, equipment control and reliable reporting. For workplace safety, ISO 45001 principles also help manage risks associated with live traffic, wet surfaces, roadside equipment and vehicle movements. International technical material can be reviewed as supplementary background, provided its methods are checked against the project’s governing standards; a technical project reference should not replace local specification requirements.

Australian Roads Require Localised Assessment

Australian road agencies operate across varied climates, traffic patterns and construction practices. Austroads guidance and state or territory road authority specifications commonly shape pavement design, surfacing, friction assessment and maintenance decisions. Project teams should confirm the required test method, reporting format and acceptance limits with the relevant asset owner rather than applying a generic threshold.

The daily driving environment also affects risk. Australian motorists frequently travel long distances, share roads with heavy vehicles and encounter sudden rain after dry periods. Motorcyclists are particularly sensitive to polished aggregate, painted markings, steel plates, raised service covers and inconsistent grip across a lane. At busy pedestrian crossings, school zones and tram or bus corridors, friction needs to be considered alongside the movement of vulnerable road users.

Urban conditions create specific testing priorities. Sydney and Melbourne have dense intersections where repeated braking can polish aggregate in the wheel paths. Brisbane and northern Australian cities may experience intense rainfall that quickly exposes drainage and texture deficiencies. In regional areas, freight traffic and seasonal surface contamination can produce different wear patterns from those found on metropolitan streets.

The legal and contractual context is equally important. Road authorities and contractors must manage public safety, workplace health and safety obligations, traffic control and duty-of-care responsibilities during investigations and repairs. A defensible test record can support decisions about temporary speed controls, surface treatment, resurfacing, warnings and maintenance programming. It can also help demonstrate that pavement risks have been identified and managed systematically.

Elevation and crossfall affect how water moves across a pavement. Localised ponding may be linked to settlement, rutting or construction tolerances rather than friction alone. Accurate vertical measurement can help compare levels, drainage paths and surface profiles when skid resistance results indicate a wet-weather problem with a geometric component.

Practical Recommendations For Safer Pavement Decisions

A skid-resistance programme works best when testing is planned around risk, not performed as a routine number-collection exercise. The following practices help produce useful results and connect them to practical maintenance action:

A new asphalt treatment should be checked after construction and again after the surface has been exposed to representative traffic. Early testing can identify flushing, poor aggregate distribution, inadequate texture or construction contamination. Later monitoring shows whether the treatment is retaining its friction performance under real traffic and weather conditions.

Corrective action may include high-pressure cleaning, removal of rubber deposits, surface texturing, resealing, resurfacing, improved drainage or a change in aggregate and mix design. The appropriate response depends on the cause. Applying a texture treatment to a pavement with underlying rutting or ponding may provide only temporary benefit, while resurfacing a clean and serviceable surface may be unnecessary if contamination is the primary issue.

A clear report should state the measured values, test limitations, site observations and recommended next steps. It should distinguish between immediate safety controls and longer-term capital works. This helps road owners allocate budgets based on evidence and allows contractors to verify whether a treatment has delivered the expected improvement.

UP. PPP supports laboratory and field investigation work for roads, bridges, drainage assets and pedestrian infrastructure through controlled testing and measurement procedures. Its technical capability can assist project teams that need documented evidence for planning, construction quality, maintenance programming or safety investigations.

When a pavement’s grip is uncertain, arrange a site assessment before the problem becomes a serious crash risk. Contact UP. PPP to discuss the appropriate field method, supporting measurements and reporting requirements for the road, bridge or pedestrian surface under review.