You may see this in a crash report, expert disclosure, or insurer letter as a number such as "drag factor 0.70," "mu = 0.45," or "coefficient of friction for the roadway surface." That number is the ratio used to measure how much resistance exists between two surfaces sliding against each other, such as a tire on asphalt, a shoe on a wet deck, or cargo shifting across a trailer floor. In accident reconstruction, it helps estimate braking performance, stopping distance, speed, and whether a vehicle or person would have slid under the conditions present.

A higher coefficient generally means more grip; a lower one means less traction. Dry pavement often produces a much higher value than rain-soaked pavement, polished metal, algae-covered surfaces, ice, or diesel-contaminated concrete. The number can come from testing, published reference tables, or calculations based on skid marks and scene evidence. Because the coefficient changes with surface material, temperature, water, frost, tire condition, and slope, small differences can materially change a reconstruction.

For an injury claim, that can affect liability, causation, and comparative fault arguments. In Maryland, where freezing rain and ice can make Beltway bridges and overpasses especially hazardous, a disputed friction value may determine whether a driver had a realistic chance to stop. If an expert uses the wrong value, the opposing side may challenge the opinion under Maryland Rule 5-702 governing expert testimony.