hills where water tends to pool. The resultant of cross slope and grade is called drainage gradient or "resulting grade". Most road design manuals require that the drainage gradient in all road sections must exceed 0.5%, in order to avoid a thick water film during and after rainfall. Areas where the drainage gradient may fall below the minimum limit 0.5% are found at the entrance and exit of banked outer curves. These hot spots are typically less than 1% of the road length, but a large share of all skid crashes occur there. One method for the road designer to reduce the crash risk is to move the cross slope transition from the outer curve and to a straight road section, where lateral forces are lower. If possible, the cross slope transition should be placed in a slight up- or downgrade, thereby avoiding that the drainage gradient drops to zero. The UK road design manual actually calls for placement of a cross slope transition in an artificially created slope, if needed. In some cases, permeable asphalt or concrete can be used to improve drainage in the cross slope transitions. Width of pavement: Wider roads require a higher cross slope to achieve the same degree of drainage. Roadway curvature Rainfall intensity and duration Vehicle sensitivity factors The driver's speed, acceleration, braking, and steering Tire tread wear: Worn tires will aquaplane more easily for lack of tread depth. Half-worn treads result in aquaplaning about 3–4 mph (5–7 km/h) lower than with full-tread tires. Tire inflation pressure: Underinflation can cause a tire to deflect inward, raising the tire center and preventing the tread from clearing water. Tire tread aspect ratio: The longer and thinner the contact patch, the less likely a tire will aquaplane. tires that present the greatest risk are small in diameter and wide. Vehicle weight: More weight on a properly inflated tire lengthens the contact patch, improving its aspect ratio. Weight can have the opposite effect if the tire is underinflated. Vehicle type: Combination vehicles like semi-trailers are more likely to experience uneven aquaplaning caused by uneven weight distribution. An unloaded trailer will aquaplane sooner than the cab pulling it. Pickup trucks or SUVs towing trailers also present similar problems. There is no precise equation to determine the speed at which a vehicle will aquaplane. Existing efforts have derived rules of thumb from empirical testing. In general, cars start to