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

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)

Roadway characteristics (affecting the drivers)

Stopping sight distance

Passing sight distance

Vertical alignment

Horizontal alignment Functional

classification

Traffic volume and mix

Topography

Pavement condition

Drainage condition

Design speed

Ch.3

Ch. 16

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.

Stopping sight distance

Stopping Sight Distance = ut + u2

2g(f ± G)

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.

Distance traveled during the PIEV time

Braking distance

Stopping Sight Distance = ut + u2

2g(α/g ± G)

OR

Decision sight distanceDSD = 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.

About Problem 3-14

10mph Speed Limit Ahead 4” high, 1” for 40ft

Readability

4*40=160ft

SSD - Readability

SSD = uit + (ui2 – uf

2)/(2g(f + G))

+ because it’s 4% upslope.

70mph 10mph

t=2.5 sec f or α/g = 0.35

With 4” letter = 160 ft

12ft

= arctan(12/160)=4.3 degs

With 8” letter = 320 ft

12ft

= arctan(12/320)=2.2 degs

Barely within the fairly clear vision cone

Inside the clear vision cone

Diversion road

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.

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.

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

Four components of PSD (cont)

d3 = empirical value d4 = 2/3 (d2)

Difference in speed

Average speed is used.

Or, simply

d1 = uit + ½*at2

It is the distance traveled with a constant acceleration rate starting at ui, the speed of the impeder.

Speed of the passer

PSD component values: Open page 77.

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

PSD: AASHTO VS MUTCD

0

500

1000

1500

2000

2500

3000

30 35 40 45 50 55 60 65

85th percentile speed or passing speed

PS

D i

n f

ee

t

AASHTO MUTCD Power (AASHTO) Power (MUTCD)

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.