4B. Sight distance considerations

Question 1

Where can decision sight distance be found

Answer 1

AASHTO: A policy on Geometric Design (Green Book): Chapter 3, subsection 3.2.3

Question 2

Where can a table with decision sight distances be found?

Answer 2

AASHTO: A policy on Geometric Design (Green Book): Chapter 3, Table 3-3

Question 3

On Table 3-3: Decision Sight Distance, what are the differences between Avoidance Maneuvers A & B, and the Avoidance Maneuvers C, D, and E.

Answer 3

Avoidance Maneuvers A & B involve the vehicle coming to a stop.
Avoidance Maneuvers C, D, and E involve a Speed/path/direction change

Question 4

Where can a formula be found for decision sight distances?

Answer 4

AASHTO: A policy on Geometric Design (Green Book): Chapter 3, Equation 3-4 and Equation 3-5

Question 5

Which formula would be used for determining decision sight distances for maneuvers A & B

Answer 5

DDS = 1.47Vt + 1.075 (V^2/a)
where:
DSD = decision sight distance, ft
t = pre-maneuver time, s
(see notes in Table 3-3)
V = design speed, mph
a = driver deceleration, ft/s2

Question 6

What are some common sight distance considerations

Answer 6

Stopping Sight Distance: AASHTO green book: 3.2.2
Decision Sight Distance: AASHTO green book: 3.2.3
Passing Sight Distance for Two-Lane Highways: AASHTO green book: 3.2.4
Sight Distance for Multilane Highways: AASHTO green book: 3.2.5
Height of Driver’s Eye
Height of Object
Sight Obstructions
Measuring Sight Distance

Question 7

What is the height of the driver’s eye above the roadway assumed to be?

Answer 7

3.5ft (1.08m)
AASHTO green book: 3.2.6.1