24 Frictional resistance is a measure of gravity’s ability to apply frictional force between asphalt surfaces and vehicle tires. This force is necessary for vehicles to accelerate and decelerate. These factors reduce frictional resistance: • The pavement’s macrotexture — this is roughness related to variations in the height of individual pieces of aggregate and the binder between them. • The pavement’s microtexture — this is the roughness of the surface of the binder and individual pieces of aggregate that tires contact. • Greater macrotexture and microtexture both correlate with greater frictional resistance. • Microtexture is reduced from the wear of repeated vehicle contact with the pavement. • Vehicle wear can also reduce the macrotexture if no additional aggregate is placed on the road surface for an extended period of time. • Bleeding (excessive binder at the surface of the pavement) reduces the frictional resistance. There are other factors that will influence the frictional force between the road and tires, such as the quality of tires and the presence of water or ice. While very important, they are not measures of the inherent friction resistance of the asphalt pavement. Undesirably low frictional resistance can be prevented by: • Using design and construction best management practices. • Applying a treatment with a texture that will restore good frictional resistance. Tips on responding to reduced frictional resistance: • Refer to the page on bleeding (page 28) if there is excessive binder at the surface of the pavement. REDUCED FRICTIONAL RESISTANCE Frictional resistance in asphalt, also known as skid resistance, is related to the interaction between a vehicle’s tires and the asphalt pavement. The pavement’s surface should be safe and comfortable for users, especially at higher speeds.
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