A motorist traveling at 55 mph on an expressway intends to exit from the express

Question

A motorist traveling at 55 mph on an expressway intends to exit from the expressway using an exit ramp with a maximum allowable speed of 30 mph. At what point on the expressway should the motorist step on her brakes in order to reduce her speed to the max allowable ramp speed? Assume the coefficient of friction between the pavement and tires is .3 and the alignment of this section of the road is horizontal.

For this particular problem we can ignore the friction value and just use a standard deceleration rate of 11.2 ft/sec.

Explanation / Answer

u = 55mph = 80.7ft/s

v = 30mph = 44ft/s

v2 - u2 = 2as

=> distance traveled while reducing the speed to 30mph = (80.7-44)/2*11.2 = 1.64 ft

so she must apply the brakes 1.64 ft before the ramp.

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