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In the UK, the typical total stopping distances (thinking distance plus braking distance) used in The Highway Code are quoted in Rule 126 as: [24] 20 mph: 40 feet (12 metres) 30 mph: 75 feet (23 metres) 40 mph: 118 feet (36 metres) 50 mph: 175 feet (53 metres) 60 mph: 240 feet (73 metres) 70 mph: 315 feet (96 metres)
The Basic Speed Law constrains the assured clear distance ahead to the total stopping distance, and the small angle value of road grades approximates the superelevation "e." Many roadways are level, in which case the small angle approximations or superelevation may be dropped altogether:
The design sight distance allows a below-average driver to stop in time to avoid a collision in most cases. Driver perception/reaction distance is calculated by: d PRT = 0.278 Vt (metric) d PRT = 1.47 Vt (US customary) Where: d PRT = driver perception-reaction distance, m (ft) V = design speed, km/h (mph) t = brake reaction time, in seconds
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 .
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.
Research in 1998 showed that the reduction of some 30 mph (48 km/h) United Kingdom speed limits to 20 mph (32 km/h) had achieved only a 1 mph (1.6 km/h) drop in speeds and no discernible reduction in accidents; 20 mph (32 km/h) speed limit zones, which use self-enforcing traffic calming, achieved average speed reductions of 10 mph (16 km/h ...
The maximum speed limit in American Samoa is 30 mph (48 km/h), with 15 mph (24 km/h) in residential areas. [3] [8] The 30 miles per hour speed limit is the lowest maximum speed limit of any state or permanently inhabited territory. Although 30 mph is the maximum speed limit, in most areas 25 mph (40 km/h) is the maximum speed limit. [9]
A typical medical consequence of rear-ends, even in collisions at moderate speed, is whiplash. In more severe cases, permanent injuries such as herniation may occur. The rearmost passengers in minivans , benefiting little from the short rear crumple zone , are more likely to be injured or killed.