Lec 6, ch3, pp7279: Road Characteristics Sight Distances Objectives

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)
Ch.3
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 highways
design speed, adequate time is given, after an
object is observed in the vehicles 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 vehicles path without hitting that object.
Distance traveled during the PIEV time
OR
Braking distance
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
because its 4 upslope.
f or a/g 0.35
t2.5 sec
SSD uit (ui2 uf2)/(2g(f G))
10mph Speed Limit Ahead
70mph
10mph
Diversion road
4 high, 1 for 40ft
Readability
SSD - Readability
440160ft
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.

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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
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Four components of PSD (cont)
Difference in speed
Average speed is used.
Speed of the passer
Or, simply d1 uit ½at2 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. 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.