1. Lec 6, ch3, pp72-79: Road Characteristics (Sight Distances) (Objectives)
Know the definitions of different types of sight distances
Learn by heart the SSD formula
Know how to use SSD formula in various sight distance related problems
Understand the components of passing sight distance

2. What we going to cover in class…
Roadway characteristics
Definition of “sight distance”
Stopping sight distance (review)
Stopping sight distance vs. decision sight distance
Passing sight distance
AASHTO PSD (design) vs. MUTCD PSD (operation)

3. Roadway characteristics (affecting the drivers)
Passing sight distance
Stopping sight distance
Ch. 16
Topography
Drainage condition
Pavement condition
Vertical alignment
Traffic volume and mix
Horizontal alignment
Functional classification
Design speed

4. Sight distance
Sight distance: The length of the roadway a driver can see ahead at any particular time.
The sight distance has to be long enough such hat when a driver is traveling at the highway’s design speed, adequate time is given, after an object is observed in the vehicle’s path, to make the necessary evasive maneuvers without colliding with the object.

5. Stopping sight distance
SSD = Minimum sight distance required for a driver to stop a vehicle after seeing an object in the vehicle’s path without hitting that object.
Stopping Sight Distance = ut + u2
2g(f ± G) OR
Distance traveled during the PIEV time
Braking distance
Stopping Sight Distance = ut + u2
2g(α/g ± G)

6. Decision sight distance
DSD = Distance required for a driver to detect an unexpected or otherwise difficult-to-perceive information source or hazard in a roadway environment that may be visually cluttered, recognize the hazard of its threat potential, select an appropriate speed and path, and initiate and complete the required safety maneuvers safely and efficiently.
Compare Table 3.4 (SSD) and Table 3.5 (DSD) values. DSD is longer than SSD because of the reasons discussed above.

7. About Problem 3-14 SSD = uit + (ui2 – uf2)/(2g(f + G))  
f or α/g = 0.35
+ because it’s 4% upslope.
t=2.5 sec
SSD = uit + (ui2 – uf2)/(2g(f + G))
70mph
10mph Speed Limit Ahead
10mph
Diversion road
4” high, 1” for 40ft
Readability
SSD - Readability
4*40=160ft
Barely within the fairly clear vision cone
Inside the clear vision cone
With 4” letter = 160 ft
With 8” letter = 320 ft  
12ft
12ft
 = arctan(12/160)=4.3 degs
 = arctan(12/320)=2.2 degs

8. Passing sight distance (AASHTO definition)
PSD = Minimum sight distance required on a two-lane, two-way highway that will permit a driver to complete a passing maneuver without colliding with an opposing vehicle and without cutting off the passed vehicle. It will also allow the driver to successfully abort the passing maneuver if desired.
Only single passes are considered.
And usually the case where a passenger car passes a passenger car is assumed.

9. Five assumptions for PSD
The impeder is traveling at a uniform speed.
The passer is traveling at the same speed as the impeder until he decides to pass.
On arrival at a passing section, some time elapses to decide to pass.
If the decision is made, the passer accelerates and the average passing speed is about 10 mph more than that of the impeder (e.g. if traveling at 30 mph, the target passing speed is 40 mph)
A suitable clearance exists between the passer and the opposing vehicle when the pass is completed.

10. Four distance components of PSD
Distance traveled during PRT & initial acceleration
The passing vehicle is in the left lane
Safety clearance
The opposing vehicle traveled during 2/3 of d2

11. Four components of PSD (cont)
Difference in speed
Average speed is used.
Speed of the passer
Or, simply
d1 = uit + ½*at2
It is the distance traveled with a constant acceleration rate starting at ui, the speed of the impeder.
d3 = empirical value
d4 = 2/3 (d2)

12. PSD component values: Open page 77.

13. PSD: AASHTO (for design of new high-standard highways) vs
PSD: AASHTO (for design of new high-standard highways) vs. MUTCD (for the operation of the existing low-standard roads)

14. From my research with Garber…
My data showed that the drivers use longer passing distance (i.e. longer time) when available sight distance is longer. Most likely, they are more relaxed.