Capacity & level of service (transportation engineering)

1. 1
Transportation Engineering
(CE-421)
Highway Capacity & LOS

2. Design Speed
2
AASHTO defines design speed as follows:
“Design speed is a selected speed used to determine the
various geometric features of the roadway.
Or
“The maximum safe speed that can be maintained over a
specified section of highway when the conditions are so
favorable that the design features of the highway govern”
Or
The service flow rate that is used for design of specific
road facility is known as Design Capacity

3. Design Speed
3
The assumed design speed should be a logical one with
respect to the;
• Topography (Terrain)
• Anticipated operating speed
• The adjacent land use
• Functional classification of the highway.
• Terrain Types?
• Level, Rolling, Mountainous,?
• Pedestrian?

4.  In selection of design speed, every effort should be made to attain
a desired combination of safety, mobility, and efficiency within the
constraints of ,
 environmental quality
 Economics
• Aesthetics, and social or political impacts.
 Once the design speed is selected, all of the pertinent highway
features should be related to it to obtain a balanced design.
Design Speed
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5.  Capacity is defined as the maximum number of vehicles,
passengers, or the like, per unit time, which can be accommodated
under given conditions with a reasonable expectation of
occurrence
Design Capacity
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 Capacity is independent of the demand. It speaks about the
physical amount of vehicles and passengers a road can afford. It
does not depend on the total number of vehicles demanding
service.
 On the other hand, it depends on traffic conditions, geometric
design of the road etc. For example,
a curved road has lesser capacity compared to a straight road.
Capacity is expressed in terms of units of some specific thing (car,
people, etc.), so it also does depend on the traffic composition.

6.  1. Traffic conditions:
It refers to the traffic composition in the road such as the mix of
cars, trucks, buses etc in the stream. It also include peaking
characteristics, proportions of turning movements at intersections
and the like.
 2. Road way characteristics:
This points out to the geometric characteristics of the road. These
include lane width, shoulder width, lane configuration, horizontal
alignment and vertical alignment.
 3. Control conditions: This primarily applies to surface facilities and
often refer to the signals at intersections etc.
Design Capacity
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7. Factors Affecting Highway Capacity
1. Lane width
2. Width of shoulder
3. Lateral clearance
4. Commercial vehicles
5. Road alignment and geometry (curves, Super elevation etc)
6. Existence of intersections.
7. One way or two way traffic and number of lanes
8. Drivers and vehicular characteristics
9. Single type or mixed traffic
10. Flow speed
11. Weather condition
12. Road Side Parking
13. Presence of pedestrians
Design Capacity
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8. Flow Rate/ Hourly Volume
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rate of flow  q = number of vehicles during observation veh/h
observation time
 Flow is one of the most common traffic parameters. Flow is
the rate at which vehicles pass a given point on the roadway, and is
normally given in terms of vehicles per hour.

9. 9
Various Types of Traffic Volumes
Daily Volume
Average Annual Daily Traffic (AADT)
Average Annual Weekday Traffic (AAWT)
Average Daily Traffic (ADT)
Average Weekday Traffic (AWT)
 Hourly Volume
 Subhourly Volume
 Peak Hour Volumes

10. Types of Traffic Volumes
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Average Annual Daily Traffic (AADT)
Average 24-hour traffic volume at a location over a
full 365-day year, which is the total number of
vehicles passing the location divided by 365.
Average Annual Weekday Traffic AAWT)
Average 24-hour traffic volume on weekdays over a
full year, which is the total weekday traffic volume
divided by 260

11. Types of Traffic Volumes
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Average Daily Traffic (ADT)
Average 24-hour traffic at a location for any
period less than a year (e.g. six months, a
season, a month, a week or even two days)
Average Weekday Traffic (AWT)
Average 24-hour traffic volume on weekdays
for any period less than a year

12. Peak Hour Volume
12
Peak hour volumes are sometimes estimated from
projections of the AADT.
Sometimes PHV is referred as “directional design hour
volume”
(DDHV) DDHV = AADT * K * D Where:
K – proportion of daily traffic occurring during the peak
hour
D – proportion of peak hour traffic traveling in the peak
direction of flow

13. Sub Hour Volume
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14. Peak Hour Factor
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15. Peak Hour Factor
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16. Capacity & LOS
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Capacity analysis: tries to give a clear
understanding of how much traffic a given transportation facility
can accommodate.
Level of service:tries to answer how good is
the present traffic situation on a given facility. Thus it gives a
qualitative measure of traffic, where as capacity analysis gives a
quantitative measure of a facility.

17. Level of Service
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18. Ideal Conditions For LOS Determination
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Chapter 21 of the Highway Capacity Manual For rural and suburban
multilane highways Assumptions (Ideal Conditions, all other
conditions reduce capacity):
 Only passenger cars
 No direct access points
 A divided highway
 FFS > 60 mph

19. Base Conditions For LOS Determination
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Base Conditions For Freeways
 use BFFS = 70 mi/h (110 km/h) for freeways in urban areas
 use BFFS = 75 mi/h (120 km/h) for freeways in Rural areas
 6 ft. (1.8 m) minimum right shoulder clearance
 2 ft. ( 0.6 m) minimum median lateral clearance
 12 ft. (3.6m ) minimum lane width
 Only passengers cars in traffic stream
 Five or more lane in each traffic stream
 2 mi (3.2 km) or greater interchange spacing
 Drivers should be road familiar

20. Base Conditions For LOS Determination
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Base Conditions For Multilane Highways OR Highways
 use BFFS = 60 mi/h (100 km/h) for highways in urban areas
 use BFFS = 65 mi/h (105 km/h) for highways in Rural areas
 Use BFFS = Posted speed in mi/h + 5 mi/h
 12 ft. (3.6 m) minimum lane width
 12 ft. (3.6 m) minimum total lateral clearance from outside objects
(right shoulder and median) in the travel direction
 Only passenger cars in the traffic stream
 No direct access points in along the roadway
 Divided highway
 Drivers should be familiar roadway users

21. Example (LOS Determination)
21
Determine the level of service (LOS) for a
divided 4-lane highway for the following
parameters.
BFFS= 60 mph, LW = 10 ft, TLC = 10 ft
20access points/ mile, for 10% heavy trucks
on rolling terrain, base volume is 2,500
veh/hour in one direction , PHF = 0.9, Non-
familiar users
fp = 0.85

22. Example (LOS Determination)
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Source: HCM, 2000

23. Example (LOS Determination)
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Source: HCM, 2000
Lane Width:
• Base Conditions: 12 foot lanes
How much does use of 10-foot lanes decrease
free flow speed?
Flw = 6.6 mph

24. Example (LOS Determination)
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Lateral Clearance
Distance to fixed objects Assumes
>= 6 feet from right edge of travel lanes to obstruction
>= 6 feet from left edge of travel lane to object in median
TLC = LCR + LCL
TLC = total lateral clearance in feet
LCR = lateral clearance from right edge of travel lane
LCL= lateral clearance from left edge of travel lane
Source: HCM, 2000

25. Example (LOS Determination)
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Flc = 0.4 mph
Source: HCM, 2000

26. Example (LOS Determination)
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Source: HCM, 2000
fm: Accounts for friction between opposing directions of traffic in
adjacent lanes for undivided
No adjustment for divided, fm = 0

27. Example (LOS Determination)
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Source: HCM, 2000
Fa accounts for interruption due to access points along
the facility
Example: if there are 20 access points per mile, what is
the reduction in free flow speed?
fa = 5.0 mph

28. Example (LOS Determination)
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Source: HCM, 2000
BFFS = free flow under ideal conditions
FFS = free flow adjusted for actual conditions
From previous examples:
FFS = 60 mph – 6.6 mph - 0.4 mph – 0 – 5.0 mph =
48 mph ( reduction of 12 mph)

29. Example (LOS Determination)
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Calculate Flow Rate
 Heavy vehicles affect traffic & Slower stream
 larger fhv increases number of passenger vehicles to account for presence of
heavy trucks

30. Example (LOS Determination)
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f(hv) General Grade Definitions:
 Level: combination of alignment (horizontal and vertical) that allows
heavy vehicles to maintain same speed as pass. cars (includes short
grades 2% or less)
 Rolling: combination that causes heavy vehicles to reduce speed
substantially below P.C. (but not crawl speed for any length)
 Mountainous: Heavy vehicles at crawl speed for significant length
or frequent intervals

31. Example (LOS Determination)
31
f(hv) General Grade Definitions:
 Level: combination of alignment (horizontal and vertical) that allows
heavy vehicles to maintain same speed as pass. cars (includes short
grades 2% or less)
 Rolling: combination that causes heavy vehicles to reduce speed
substantially below P.C. (but not crawl speed for any length)
 Mountainous: Heavy vehicles at crawl speed for significant length
or frequent intervals

32. Example (LOS Determination)
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Example: for 10% heavy trucks on rolling
terrain, what is Fhv?
For rolling terrain, ET = 2.5
Fhv = _________1_______ = 0.87
1 + 0.1 (2.5 – 1)

33. Example (LOS Determination)
33
Driver Population Factor (fp)
Non-familiar users affect capacity
fp = 1, familiar users
1 > fp >=0.85, unfamiliar users
Calculate vp
Example: base volume is 2,500 veh/hour
PHF = 0.9, N = 2
fhv from previous, fhv = 0.87
Non-familiar users, fp = 0.85
vp = _____2,500 vph _____ = 1878 pc-ph-pl
0.9 x 2 x 0.87 x 0.85

34. Example (LOS Determination)
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Calculate Density
Example: for previous
D = _____1878 vph____ = 39.1 pc/mi/lane
48 mph

35. Example (LOS Determination)
35Also, D = 39.1 pc/mi/ln, LOS E

36. Example (LOS Improvement)
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What can we change in a design to provide an
acceptable LOS?
• Lateral clearance (only 0.4 mph)
• Lane width
• Number of lanes
Design Decision

37. Example (LOS Improvement)
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Source: HCM, 2000
Lane Width (Example)
How much does use of 10 foot lanes decrease free
flow speed?
Flw = 6.6 mph

38. Example (LOS Improvement)
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Recalculate Density
Example: for previous (but with 12 ft lanes and
TLC = 12 ft)
D = _____1878 vph____ = 34.1 pc/mi/lane
55 mph

39. Example (LOS Improvement)
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39
LOS = E
Now D = 34.1 pc/mi/ln, on border of LOS E

40. Example (LOS Improvement)
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Recalculate vp, while adding a lane
Example: base volume is 2,500 veh/hour
PHF = 0.9, N = 3
fhv from previous, fhv = 0.87
Non-familiar users, fp = 0.85
vp = _____2,500 vph _____ = 1252 pc/ph/pl
0.9 x 3 x 0.87 x 0.85

41. Example (LOS Improvement)
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Recalculate vp, while adding a lane
Example: base volume is 2,500 veh/hour
PHF = 0.9, N = 3
fhv from previous, fhv = 0.87
Non-familiar users, fp = 0.85
vp = _____2,500 vph _____ = 1252 pc/ph/pl
0.9 x 3 x 0.87 x 0.85

42. Example (LOS Improvement)
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Calculate Density
Example: for previous
D = _____1252 vph____ = 26.1 pc/mi/lane
48 mph

43. Example (LOS Improvement)
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LOS = D
Now D = 26.1 pc/mi/ln, LOS D (almost C)

44. Example 02 (LOS Determination)
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45. Example 02 (LOS Determination)
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