3. 2. Design Speed
Design speed is a selected speed used to determine the various
geometric design features of the roadway.
It is important to design facilities with all elements in balance,
consistent with an appropriate design speed.
Operating Speed (typically the 85th percentile speed)
Free-flow Speed (close to zero density)
Running Speed = length of highway section/running time
Design Speed (as high as practical)
3
Dr. Rizwan Memon
4. Design speed: (definition from Green Book)
“A selected speed used to determine the various geometric
design features of the roadway”.
With respect to the selection of appropriate design speed
AASHTO states that
“ the assumed design speed should be a logical one with respect
to the topography, anticipated operating speed, the adjacent
land use and functional classification of the highway, and once
selected all the pertinent features of the highway should be
related to the design speed”.
The speed selected for design should fit the travel desires and
habits of nearly all drivers, the design speed chosen should be a
high-percentile value.. nearly all inclusive…whenever is feasible.
4
Dr. Rizwan Memon
5. Operating speed
is a general term typically used to describe the actual speed
of a group of vehicles over a certain section of roadway.
Definition by GREEN BOOK:
the speed at which drivers are observed operating their vehicles
during free flow conditions. The 85th percentile of observed
speeds is the most frequently used measure of the operating
speed associated with a particular location or geometric feature.
5
Dr. Rizwan Memon
6. What will happen if your design speed is
30 mi/hr. for this highway?
Source of picture:
http://www.fotosearch.com/
70 mi/hr. for this?
6
Dr. Rizwan Memon
7. 7
Design Speed Considerations
Functional classification of the highway
Character of the terrain
Density and character of adjacent land
uses
Traffic volumes expected to use the
highway
Economic and environmental considerations
Dr. Rizwan Memon
8. 8
Design Speed in Green Book
(suggested minimum design speed)
Rural Collectors
Source: A Policy on Geometric Design
of Highways and Streets (The Green
Book). Washington, DC. American
Association of State Highway and
Transportation Officials, 2001 4th Ed. Dr. Rizwan Memon
10. The largest vehicle that is likely to use the facility
with considerable frequency
a) Passenger Cars, buses, trucks
b) Physical Characteristics: weight, dimensions
c) Establish intersection radius, pavement markings
Vehicle Performance
a) Operating characteristics; acc, dec., T.radius, braking
b) Impacts air quality, Noise, Dust, Fuel Efficiency
3.Design Vehicles
10
Dr. Rizwan Memon
15. driver most expected to use facility
(familiar or unfamiliar)
a) Physical Characteristics
b) Processing Ability
c) Others: age, gender, physical condition, mental
capabilities, skill
d) Perception-Reaction Time, and expectancy
c) Speed
d) Driver Error
4.Design Driver
15
Dr. Rizwan Memon
17. Design criteria must be based
on the capabilities and
limitations of most drivers and
pedestrians
The 85th percentile is generally used to select Design
Criteria
17
Dr. Rizwan Memon
18. Volume: the total number of vehicles that pass
over a given point or section of a lane or a roadway
during a given period of time (annual, monthly,
weekly, daily, or hourly).
Flow rate: The equivalent hourly rate at which
vehicles pass over a given point of lane or roadway
during a given interval less than 1 hour, usually 15
min.
5. Design Traffic Volume
18
Dr. Rizwan Memon
19. Capacity:
Maximum hourly rate of vehicles or persons that
can reasonably be expected to pass a point, or
traverse a uniform section of lane or roadway,
during a specified time period under prevailing
conditions (traffic and roadway)
Different for different facilities (freeway,
multilane, 2-lane rural, signals)
19
Dr. Rizwan Memon
23. Average Annual Daily Traffic (AADT) is the average 24-hour volume at
a given location over a full 365-day year, i.e., the total number of
vehicles passing through the observation station divided by 365 days.
AADT
Traffic Characteristics
23
Dr. Rizwan Memon
25. Traffic Characteristics (Cont’d)
Directional distribution:
For two-way rural highway, the DHV is total traffic in both
directions of travel, in the design of 2+ lane highways, directional
distribution of traffic needs to be considered.
Directional distribution factor
25
Dr. Rizwan Memon
26. 26
Design Volume
Usually hourly volume
Which hour?
Average hourly volume – inadequate design
Maximum peak hour – not economical
Hourly volume used for design should not be
exceeded very often or by very much
Usually use 30th highest hourly volume of the
year
On rural roads 30 HHV is ~ 15% of ADT
Tends to be constant year to year
Dr. Rizwan Memon
27. Characteristics of 30HV
30HV is indeed the ‘knee’ of the curve
Indicating:
1. Only few hours higher than the ‘knee’
Point
2. Curve flattens to the right, most of
the Hours lower than the ‘knee’ point
(the lowest is only 23 percent less
than 30HV and the highest point is
67% higher Than 30HV)
3. Statistically stable
Average fluctuation
27
Dr. Rizwan Memon
28. Subhourly flow fluctuation &
Peak Hour Factor (PHF)
Subhourly variation: present
short-term flow fluctuation;
For most practical purposes,
15 minutes is considered to be
the minimum period of time
over which traffic conditions
are statistically stable.
PHF (Peak Hour Factor):
defines the relationship
between the hourly volume and
the maximum rate of flow
within the hour.
PHF = Vhr./(Vmax15min * 4) 28
Dr. Rizwan Memon
29. Peak-hour volume: 1200 vehicles
PHF = 0.8
Design Hourly Volume = 1200/PHF = 1500 veh/hr.
1200
Practical use of PHF:
29
Dr. Rizwan Memon
30. Use of 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)
30
Dr. Rizwan Memon