2. 5.1 Introduction
5.2 Determining capacity and LOS of highways
4.2.1 Service Flow Rate and Service Volume Analysis
4.2.2 Basic Relationship for Capacity Analysis
3. The term level-of-service is closely related to highway capacity
and often confused with it is service volume.
When capacity gives a quantitative measure of traffic, level of
service (LOS) tries to give a qualitative measure.
The capacity of a highway is the maximum hourly flow rate at
which vehicles or persons can reasonably be expected to traverse
a point or uniform section of a lane or roadway during a given
time period under prevailing roadway, traffic, and control
conditions.
A 15 min time period is normally used. The roadway conditions
are associated with the geometric characteristics and design
elements of the facility, which include type of facility, number of
lanes, lane width, shoulder width, horizontal and vertical
alignments, lateral clearance, design speed, and availability of
queuing space at intersections.
4. Level of service (LOS) qualitatively measures both the
operating conditions within a traffic system and how
these conditions are perceived by drivers and passengers.
LOS is related with the physical characteristics of the
highway and the different operating characteristics that
can occur when the highway carries different traffic
volumes.
Speed-flow-density relationships are the principal factor
affecting the level of service of a highway segment under
ideal conditions.
5. Highway capacity manual (HCM) divides the quality of traffic into
six levels ranging from level A to level F.
Level of Service - A.
Free-flow traffic with individual users
virtually unaffected by the presence of
others in the traffic stream.
Only the geometric design features
of the highway may limit the speed of the car.
Level of Service - B.
Reasonably free flow.
Ability to maneuver is only
slightly restricted.
Effects of minor event still easily
absorbed.
6. Level of Service - C.
Speeds at or near free flow speed
Freedom to maneuver is
noticeably restricted
Queues may form behind any
significant blockage.
Level of Service - D.
Speeds decline slightly with increasing flows.
Density increases more quickly.
Freedom to maneuver is more
noticeably limited.
Minor incidents create queuing.
7. Level of Service - E.
Operation near or at capacity.
No usable gaps in the traffic stream.
Operations extremely volatile.
Any disruption causes queuing.
Level of Service - F.
Breakdown in flow.
Queues form behind
breakdown points.
Demand > capacity.
9. Factors affecting level of service,
Speed and travel time
Traffic interruptions/restrictions
Freedom to travel with desired speed
Driver comfort and convenience
Operating cost.
Lane width,
Lateral obstruction,
Traffic composition,
Grade and
Driver population
10. Highway Capacity Manual (HCM) used travel speed and
volume by capacity ratio (v/c ratio) to distinguish between
various levels of service.
The value of v/c ratio can vary between 0 and 1.
11. Hourly volume (V):- The highest hourly volume within a 24-hour
period
Peak-hour factor (PHF):- The ratio of the hourly volume to the
peak 15 minute flow (V15) enlarged to an hourly value
PHF = V ÷ (V15 × 4) = V/SF
Service flow (SF):- The peak 15 minute flow (V15)X 4
SF = V15 × 4 = V/PHF
Fig. Illustration of level of service and service flow rate
SFA SFB SFC SFD SFE
A B C D E F
12. 5.2.1 Service Flow Rate and Service Volume Analysis
The service flow rate for level of service ‘i’ is the maximum
flow rate that can be maintained under prevailing condition.
The service flow rate must be obtained by adjusting the
maximum service flow (MSFi) to reflect the number of lanes
and the prevailing conditions.
The maximum service flow rate at level of service i (MSFi) is
the maximum flow that a section of the freeway can maintain
at level of service ‘i’ under ideal conditions.
The maximum service flow rate is determined as
Where
→ MSFi = maximum service flow rate per hour per lane
(pc/hr/ln) under ideal conditions for level of service i.
13. → (V/C)i = maximum volume-to-capacity ratio for level of
service i.
→ Cj = capacity under ideal conditions for the freeway segment
having design speed j.
Service flow rate is calculated as :-
Where
fW = factor to adjust for the effect of restricted lane widths and/or
lateral clearance.
fHV = factor to adjust for the combined effect of heavy vehicles in
the traffic stream.
fp = factor to adjust for the effect of recreational or unfamiliar
driver populations.
14. Service volumes over the full peak hour
The following tables gives maximum service flow rates,
maximum density, and minimum speed for different free-flow
speeds at levels of service A-E. Since operating at level of
service E is the same as operating at capacity, the maximum
service flow rate at level of service E equals the capacity of
the freeway or multilane segment.
19. Adjustments to Maximum Service Flow Rate.
a. Restricted Lane Width and Lateral Clearance Factor,
(fw)
Used to adjust for lane widths less than 12 ft and/or lateral
clearance less than 6 ft.
20. b. Heavy Vehicle Adjustment Factor; (fHV)
Based upon the concept of passenger-car equivalents.
A passenger-car equivalent is the number of passenger
cars displaced by one truck, bus, in a given traffic stream
under prevailing conditions.
Where:
→ PT = proportion of trucks and buses in the traffic stream,
→PR = proportion of RV s in the traffic stream.
→ ET = passenger car equivalent for trucks and buses,
→ ER = passenger car equivalent for RVs.
21. Passenger-Car Equivalents for Trucks, Buses, and RVs on
Extended General Terrain Sections of Freeways or
Multilane Highways.
26. EXAMPLE (fHV)
Consider the following situation: A volume of 2,500 veh/h
traverses a section of freeway and contains 15% trucks and 5%
RVs. The section in question is on a 5% upgrade, 0.75miles in
length. What is the heavy vehicle adjustment factor and the
equivalent volume in passenger car equivalents?
Solution:
The solution is started by finding the passenger car equivalent
of trucks and RVs on the freeway section described (5%
upgrade, 0.75 miles).
These are found in Tables before, respectively:
→ ET = 2.5 ( 15% trucks, >4-5%, >0.50-0.75 mi) and ER = 3.0
( 5% RV's, >4-5%, >0.50 mi)
27. → fHV = 0.7547
The passenger-car equivalent volume may be estimated
as:
The solution can also be found by applying the passenger
car equivalents directly:
Truck pces: 2,500 * 0.15 * 2.5 = 938
RV pces: 2,500*0.05*3.0= 375
Pass Cars: 2,500 * 0.80 * 1.0 = 2,000
TOTAL pees: 3,313
28. c. Driver Population Adjustment Factor; (fp)
It is correction for the case when non commuter drivers
are prevalent in the traffic stream.
Not well defined and is dependent upon local conditions.
On some recreational routes, the majority of drivers may
not be familiar with the route.
Adjustment Factor for Driver Population .
29. 5.2.2 Basic Relationship for highway Capacity
Analysis
There are two types of problems that are solved by
capacity analysis. They are:
Type I: Given the highway volume and the number of
lanes, determine the maximum service flow rate and the
level of service.
Type II: Given the highway volume and the level of
service, determine the number of highway lanes required.
To solve these problems, it is necessary to convert the
given highway volume to equivalent 15-minute peak-hour
volume, which is computed as follows.
30. Ve = v/PHF
where: Ve = equivalent15-min peak-hour volume (vph)
v = actual hourly volume (vph)
PHF = actual hourly volume divided by 4 times the peak 15-
min volume
SFi = v/PHF = MSFi(N)(fw)(fHV)(fp)
And
MSFi = v/ ((N)(fw)(fHV)(fp) (PHF))
To compute the number of lanes:
N = v/ ((MSFi)(fw)(fHV)(fp) (PHF))
31. EXAMPLE
A six way urban freeway on a rolling terrain has a free flow
speed of 60mi/hr and a PHF of 0.92. The current peak hour
volume is 3600veh/hr which is expected to grow at a rate of 6%
per year for the next 20 years.
The traffic consists of 8% trucks and no RVs and all the drivers
are regular users of the facility(fP=1).
a) What is the capacity of the Highway?
b) What is the current LOS of the highway and the LOS’s at
5,10,15 and 20 years.
c) What is the exact year that demand reaches capacity?(the exact
year that the LOS becomes F)
32. Solution:
Step 1- Determine the MSFi for each LOS for free flow speed of
60mi/h from the tables
Step-2- determine the heavy vehicle factor (fHV)
LOS MSFi
A 660
B 1080
C 1560
D 2020
E 2300
33. Since PT= 0.08
PR=0
ET=2.5(rolling terrain)
Step 3- determine the service flow rate and the service volume for
each LOS.
N=3
PHF=0.92
fHV= 0.893
fP=1(regular users)
35. The capacity of the highway is the service volume
at LOS ‘E’ ,which is 5669veh./h
LOS MSF N fHV fP SF
(veh./h)
PHF SV
(veh./h)
A 660 3 0.893 1 1768 0.92 1627
B 1080 3 0.893 1 2893 0.92 2661
C 1560 3 0.893 1 4179 0.92 3844
D 2020 3 0.893 1 5411 0.92 4978
E 2300 3 0.893 1 6162 0.92 5669
37. To calculate the exact year that the demand reaches capacity
5669=
Thus n= 7.8 years.
The highway will reach its full capacity at 7.8 years and the LOS of
the road after that will be ‘F’.
