Question

A child accidentally runs into the street in front of an approaching vehicle. The vehicle is travelling at 50 mi/hour. Assuming the road is level, at what distance must the driver first see the child to stop just in time?a) 153.7 ftb) 300.3 ftc) 318.8 ftd) 146.7 ft

Answer 1

Answer:

b) 300.3 ft if the vehicle is travelling at 40 mi/hour

   423.4 ft if the vehicle is travelling at 50 mi/ hour

Explanation:

First Step:

Calculate the distance traveled during braking for level road () using the formula:

Deceleration rate is considered 11.2 ft/s² (AASHTO recommendations)

a=11.2 ft/s²

=1^2/2=(50 mi/h×5280 ft/1 mi×1 h/3600 s)²/22×11.2=240.07 ft

2nd Step:

Calculate distance traveled during perception /reaction (dr ) using the formula:

Standard Reaction time is 2.5 sec ( AASHTO )

= 2.5 sec

=1×=(50 mi/h×5280 ft/1 mi×1 h/3600 s)×2.5=183.33 ft

Third Step:

Stopping distance would be:

=+=240.07+183.33 = 423.4 ft

Thus, the correct answer is b if the vehicle is travelling at 40 mi/hour