Road texture

Content sourced from Wikipedia, licensed under CC BY-SA 3.0.

Road texture describes the tiny bumps and patterns on a road surface that change how tires grip it. It is usually split into three categories: microtexture, macrotexture, and megatexture.

- Microtexture (0 to 0.5 mm): These are very small surface imperfections that influence friction at the tire–road contact. They come from the material’s microstructure and are hard to measure directly; friction tests are often used instead.

- Macrotexture (0.5 mm to 50 mm): These are larger features, like waves or grooves a few millimetres to centimetres apart. Short macrotexture waves (around 5 mm) can reduce tire noise, while longer waves can increase noise. Macrotexture helps with grip on wet roads but too much can raise rolling resistance and fuel use. A common target is about 1 mm mean profile depth.

- Megatexture (50 mm to 500 mm): Very large surface features caused by wear or distress, such as potholes or washboard textures. These raise noise and vibration and signal road damage. Normal megatexture is typically below 0.2 mm RMS on good roads.

Measurement and use:
Road texture is measured with profilometers and profilographs that use lasers and inertial sensors to record surface elevations. From these measurements, the International Roughness Index (IRI) is calculated. IRI ranges from 0 (very smooth) to several hundred (very rough) and helps road authorities monitor safety and quality. Profilers can also measure cross slope, curvature, rutting, cracks, and even asphalt thickness with radar.

Practical notes:
- Concrete roads are often grooved to improve friction.
- Asphalt roads with gravel or seal coats can have higher macrotexture.
- Cross slope and curvature, together with texture, affect tire forces during cornering and can help identify unsafe banked curves.

In short, microtexture, macrotexture, and megatexture describe different scales of road roughness, each influencing grip, noise, and fuel use in its own way.