Design Controls.

1. Dr. Rizwan Memon 1

2. Highway Geometric Design Control
Factors
1. Highway Classification
2. Design Speed
3. Design Vehicle
4. Design Driver
5. Design Traffic Volume
2
Dr. Rizwan Memon

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

9. 9
Design Speed in Green Book
(suggested minimum design speed)
Rural Arterials
 60 – 120 kmph (40-75 mph)
 Depends on …
 Terrain
 Driver expectancy
 Alignment (reconstruction)
Urban
 Collectors 30 mph+ (48 kmph +)
 Locals 20-30 mph (32 – 48 kmph)
 Arterials 30-60 mph
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

11. Vehicle dimension and
Turn radius
e.g. Passenger car
11
Dr. Rizwan Memon

12. Truck
12
Dr. Rizwan Memon

13. 13
Dimensions of Design Vehicles
Dr. Rizwan Memon

14. 14
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

16. 0
2
4
6
8
10
12
<
20
20-24
25-29
30-34
35-39
40-44
45-49
50-54
55-59
60-64
65-69
70-74
75-79
80-84
>
84
Percent
of
Driving
Population
Age Groups
16
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

20. 20
0
1000
2000
3000
4000
5000
6000
7000
Flow
in
vehicles
per
hour
Time of Day
Highway Capacity
Highly Congested
Traffic Typically Peaks twice per day
Source: www.ecn.purdue.edu/~darcy Dr. Rizwan Memon

21. 21
Traffic Volume Studies
 AADT: Annual average daily traffic
(counted for 365 days)
 ADT: average daily traffic (counted
for > 1 day and < 365 days)
 PHV: Peak Hour Volume
 Classification counts: fleet mix
Dr. Rizwan Memon

22. Traffic Characteristics
Volume:
Monthly variation
22
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

24. Traffic Characteristics (Cont’d)
60
70
80
90
100
110
120
130
140
150
Sun. Mon. Tues. Wed. Thurs. Fri. Sat.
Rural recreational highway
Main rural highway
Urban freeway
Annual
average
daily
traffic
as
percent
of
AADT
weekly variation
AADT
24
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