skid

1. Skid resistance, friction
and texture
Dr. Vidhi Vyas
Scientist
Pavement evaluation division

2. Surface friction
• Surface friction, alternatively referred to as skid resistance,
affects the braking ability of vehicles, hence it is the main safety-
related attribute of pavement surface condition.
• In elementary physics terms, friction can be indexed by the
coefficient of friction f , defined as the ratio of the shear force S,
generated as the result of slippage between two bodies, divided
by the normal force N exerted between them:
f = S/N

3. Skid resistance

4. • where SN = skid number; F = traffic force (horizontal force applied
to the test tire at the tire–pavement contact patch, Newton or lb;
W = dynamic vertical load on the test wheel, Newton or lb.

5. Factors:
• Vehicle speed
• Macro-texture
• Surface condition
• Tire characteristics
• Weather
• Road geometry
• Micro-texture

6. Tests and apparatus

7. Texture is differentiated into microtexture (wavelengths in
the range of 1 μm to 0.5mm) and macrotexture
(wavelengths in the range of 0.5mm to 50 mm).
Microtexture contributes to skid resistance at low traffic
speed, while macrotexture governs skid resistance at high
speed.

8. Micro-texture

20. Macro-texture