Improved method for determination of braking distance on downgrade pavement slope

In calculating vehicle braking distances on downgrade slopes, the commonly used method of adding a slope correction factor to the level-road formula is approximate, with unknown errors to users. This study proposes an improved method with three key refinements: (i) variation of pavement skid resistance with speed during braking; (ii) influence of water film thickness on skid resistance; and (iii) reduction in tire-pavement friction due to lower normal tire load because of the downgrade slope. These factors are incorporated using a finite element simulation model to derive the skid resistance – speed relationship and a numerical integration procedure for braking distance calculation. Applied to downgrade slopes up to 28%, the method shows braking distance increases with both slope and water film thickness. Traditional methods assuming constant friction or deceleration fail to capture these effects. The proposed method provides more reliable predictions of braking distances under wet conditions on slopes.