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Braking distance refers to the distance a vehicle will travel from the point when its brakes are fully applied to when it comes to a complete stop. It is primarily affected by the original speed of the vehicle and the coefficient of friction between the tires and the road surface, [Note 1] and negligibly by the tires' rolling resistance and vehicle's air drag.
d MT = braking distance, m (ft) V = design speed, km/h (mph) a = deceleration rate, m/s 2 (ft/s 2) Actual braking distances are affected by the vehicle type and condition, the incline of the road, the available traction, and numerous other factors. A deceleration rate of 3.4 m/s 2 (11.2 ft/s 2) is used to determine stopping sight distance. [6]
For heavy duty commercial vehicles it is recommended 4-6 seconds following distance for speeds under 30 mi/h (48 km/h), and 6-8 seconds following distance for speeds over 30 mi/h (48 km/h). [9] Rear-end collisions are the number one type of traffic collisions .
British Railway Class 90 infobox showing brake force Brake force to weight ratio of the Class 67 is higher than some other locomotives. In the case of railways, it is important that staff are aware of the brake force of a train so sufficient brake power will be available to bring the train to a halt within the required distance from a given speed.
The most important design criterion for these curves is stopping sight distance. [2] This is the distance a driver can see over the crest of the curve. If the driver cannot see an obstruction in the roadway, such as a stalled vehicle or an animal, the driver may not be able to stop the vehicle in time to avoid a crash.
Safe driving requires that the stopping distance always be less than the sight distance. In determining the safe speed limit for a given road, or designing a road for a given speed, assumptions must be made for the reaction speed of a driver and how much a vehicle can decelerate, using safety margins, while the minimal coefficient of friction ...
On the M25 for example, distances are referenced to a point near Junction 31 even though the section of the ring road between Junction 31 (post 186.6) and Junction 1a (post 5.7) is the A282, not the M25. [3] On motorways distance marker posts also bear an arrow pointing towards the location of the nearest emergency telephone. [4]
The two-second rule is useful as it can be applied to any speed. Drivers can find it difficult to estimate the correct distance from the car in front, let alone remember the stopping distances that are required for a given speed, or to compute the equation on the fly. The two-second rule provides a simpler way of perceiving the distance.