Transcript Geometric Design (I) Learning Objectives • To understand the considerations and quantifiable aspects of geometric design.

Geometric Design (I)
Learning Objectives
• To understand the considerations and
quantifiable aspects of geometric design
Locational Design
• Consider
– Current land use
– Geology
– Future land use
– Existing infrastructure
Controls and Criteria (1)
• Design Vehicles (p. 168-173)
– Passenger cars, buses, trucks, RVs
– Physical characteristics: weight, dimensions
– Establish intersection radius, pavement markings
• Vehicle Performance
– Operating characteristics: accel/decel
– Impacts air quality, noise, land use
Controls and Criteria (2)
• Driver
– Information handling
– Reaction time
• Time to perceive + react to a hazard in vehicle’s path
• Expected/unexpected
– Speed
– Driver errors
Controls and Criteria (3)
• Traffic
– Composition and volume
• Average daily traffic (ADT) is not adequate
• Design hourly volume (DHV)
• 30th-highest hourly volume (30HV) in one year
• K-factor (% of ADT; 8~12% urban, 12~18% rural)
– Speed
• Operating Speed (typically the 85th percentile speed)
• Free-flow Speed (close to zero density)
• Running Speed (actual speed)
• Design Speed (as high as practical)
Controls and Criteria (4)
• Capacity
– Maximum hourly flow rate (per lane) under
prevailing conditions
– Determines adequacy of existing roadways
– Helps select roadway type
– Helps define needs
– Design level of service
(LOS)
Stopping Sight Distance (1)
• Length of roadway that should be visible
ahead of you in order to ensure that you
will be able to stop if there is an object in
your path
• Calculate the SSD for a vehicle traveling
on your roadway at the design speed, and
then make sure the actual sight distance
that you provide is at least as great as the
stopping sight distance
Stopping Sight Distance (2)
• Assume
– Driver eye height of 3.5 feet
– Height of object between 2.0 and 3.5 feet
• Reaction distance + braking distance
2
SSD( ft )
V( mph )
 1.47 V( mph )  tr ( s ) 
 a( ft / s 2 )

30
 G
 32.2

• Design standard: tr=2.5, a=11.2
Other Sight Distances
• Decision sight distance
– Allow longer tr for information processing for
different maneuver conditions (table 6-5)
• Passing sight distance
– Ensure safe passing maneuver (figure 6-5)
– 4 distance components (figure 6-6)
• At 70 mph
– SSD = 730 ft
– DSD = 1445 ft (maneuver E)
– PSD = 2480 ft
Horizontal Alignment
• Basic controlling expression
eu 
V (2mph )
15R( ft )
e = rate of superelevation
u = side friction factor (dep. on pavement, speed, …)
V = vehicle speed
R = radius of curve
Horizontal Alignment
• Overall design procedure
– Determine a reasonable maximum
superelevation rate.
– Decide upon a maximum side-friction factor.
– Calculate the minimum radius.
– Iterate and test several different radii until you
are satisfied with your design.
– Make sure that the stopping sight distance is
provided. Adjust your design if necessary.
– Design the transition segments.
Superelevation
• Tilting the roadway to help offset centripetal
forces developed as the vehicle goes around a
curve
• General Practice
– Highways, no ice/snow
emax = 0.10
e
– Highways, snow/ice
1
emax = 0.06
– Traffic congestion or roadside development, limit
speeds
emax = 0.04 ~ 0.06
Side Friction
• Design based on point where centrifugal
force creates feeling of discomfort for
driver
Speed
umax
udesign
20
0.50
0.17
30
0.35
0.16
40
0.32
0.15
50
0.30
0.14
60
70
0.29
0.28
0.12
0.10