3A. Forgiving roadside concepts

Question 1

EstimatingSpeedofVehiclefromSkidMarks

Answer 1

P.E. Handbook 5.1.4.3

db =((v1)^2-(v2)^2) / (30 (f +-G))

db=horizontaldistancetraveled(ft)inreducingspeedofvehiclefrom v1tov2duringbrakingmaneuvers
v1=vehiclespeedwhenbrakesareapplied(initialspeed)(mph)
v2=vehiclespeedatendoftravel(finalspeed)(mph)
f=a/g =coefficient of friction (unitless) between tires and road pavement
a=decelerationofvehiclewhenbrakesareapplied(ft/sec2)
g=accelerationduetogravity(32.2ft/sec2)
G=gradeofroadway(decimalform;negativeifdownhill)

Question 2

What slopes are consider recoverable?

Answer 2

1V:4H or flatter

Question 3

What slopes are consider non-recoverable?

Answer 3

Between 1V:3H and 1V:4H

Question 4

What slopes are consider critical?

Answer 4

Steeper than 1V:3H

Question 5

What is the recommend clear zone for AASHTO “Green Book”?

Answer 5

7 to 10 ft

Question 6

What is the design clear zone a function of?

Answer 6

Design speed, side slope, and traffic volume

Question 7

Per the Green Book what is the minimum lateral offset from a curved road? At the intersection?

Answer 7

1.5 ft, 3 ft