our Compute the density of traffic per length of vehicles/km. Answer: 21.92 vehicles/km 2. Six vehicles are observed in a 200 m section of a highway. If the average time headway is 4 seconds, determine the space mean speed in kph. Answer: 30 kph 3. One vehicle is following another on a two-lane two highway at night according to the safe driving rule of thumb of one car length spacing for each 16 kph of speed. If both vehicles are traveling at 80 kph and the lead car crashes at that speed into the rear on an unlighted parked truck, at what speed will the following vehicle hit the wreckage? Assume a car length is 6 meters, reaction time is 0.5 sec. and a coefficient of friction is 0.65. Answer: 57.25 kph 4. Compute the braking distance for a car moving at an initial velocity of 60 kph and a final yelocity of 40 kph. Slope of roadway = +5% Coeff. of friction between road pavement & tires = 0.15 ion time = 3/4 sec.