Question

Sections of this interstate have speed limits of 65mph. A construction site is located on this interstate near Boise, ID. In the construction site, the speed limit is reduced to 20 mph. A warning sign reading "WORK ZONE: 20 MPH SPEED ZONE" is required to be put before the start of the construction site so as to allow drivers to decelerate before entering the work zone. The sign is located 20 ft. to the side of the road, the deceleration rate of the vehicle is 11 ft./sec2 (recommended maximum deceleration rate by AASHTO) and the driver is assumed to have 20/40 vision. You may assume a perception time of 1 sec, an interpretation rate of 3 words/sec, and a decision time of 0.5 sec. Assuming the road is level answer the following questions.

a. What should be the minimum distance away from the warning sign from where the driver is able to read the warning? (Assuming 10 degree cone of vision and the vehicle is moving on the side of the road i.e. lateral distance between car and warning sign is 20 ft.)

b. Determine the total time taken by driver before he applies brakes. Also, determine the distance travelled during perception, interpretation, and reaction.

c. Assuming that the driver reacts only once he is out of 10 degree cone of vision, calculate the minimum distance away from the start of the construction site where a warning sign must be located to allow drivers to decelerate before entering the work zone.

d. Determine the minimum size of the text on the sign.

Answer 1

Step-by-step explanation:

Given Preception time = 1.0 sec

Deceleration rate = 11 ft /sec2

Interpretation rate as = 3words /sec

The board consists of “WORK ZONE: 20 MPH SPEED ZONE”

Then Interpretation time = 2.0 sec

Decision time = 0.5 sec

Assume that the vehicle was traveling with 65mph

Perception Time

So, if you’re driving at 65 mph, your vehicle will travel 71 feet before you realize you need to start braking.

Reaction Distance

At 65 mph, that’s another 71 feet traveled.

Braking Distance

At 65 mph, it takes an additional 5.5 seconds or about 525 feet of actual brake application to stop your vehicle.

Stopping Distance

To determine the stopping distance, you calculate: Perception Distance (71 feet) + Reaction Distance (71 feet) + Braking Distance (525 feet) = Stopping Distance (667 feet)

Then to reduce the speed it takes 7.0 secs