HIGHWAY CAPACITY STUDIES. Lecture Notes in Civil Engineering

CVE 719
MATERIALS AND METHODS FOR THE
CONSTRUCTION OF HIGHWAYS
ENGR. DR. ANDERSON ETIKA
DEPARTMENT OF CIVIL ENGINEERING
CRUTECH, CALABAR
LECTURE 1

CVE 719: MATERIALS AND METHODS FOR THE
CONSTRUCTION OF HIGHWAYS
• COURSE CONTENT*
• HIGHWAY CAPACITY STUDIES
• INTERSECTIONS AND INTERCHANGE
• INTERSECTION DESIGN
• DELAY STUDIES
• ACCIDENT STUDIES
• TRAFFIC CONTROL SYSTEMS
* THIS COURSE IS CO-TAUGHT BY ENGR. DR. D. EWA, THUS THIS IS ONLY PART OF THE COURSE CONTENT
TO BE TAUGHT BY ENGR. DR. A. ETIKA*

OBJECTIVES
Review LOS definition and determinants
Define capacity and relate to “ideal”
Capacities
Review calculating capacity using HCM
procedures for basic freeway section
Focus on relations between capacity,
level of-
service, and design.
HIGHWAY CAPACITY STUDIES

INTRODUCTION
How much traffic can a given facility accommodate?
Under what operating conditions can it accommodate that
much traffic?

TYPES OF APPROACHES
There are two different approaches to determining the
capacity of a highway
1. ‘Level of Service Approach
2. Capacities for roads of various sizes and widths

LEVEL OF SERVICE
A qualitative measure describing operational conditions
within a traffic stream and their perception by drivers
and/or passengers.
It gauges the level of congestion on a highway in terms of
variables such as travel times and traffic speed.
The Highway Capacity Manual (TRB, 1994) list 6 levels of
services ranging from A (best) to F (Worst).
Levels represent range of operating conditions defined by
measures of effectiveness (MOE).

LOS A (FREEWAY/MOTORWAY)
 Free flow conditions (traffic flow is
virtually zero)
 Vehicles are unimpeded in their ability
to manoeuver within the traffic stream.
 Incidents and breakdowns are easily
absorbed
Comfort and convenience levels are high

LOS B
 Reasonable free-flowing conditions
Ability to manoeuver is slightly
restricted
General level of physical and
psychological comfort provided to
drivers are still high
Effects of incidents and breakdowns
are easily absorbed

LOS C
 Delivers stable flow conditions
Freedom to maneuver is noticeably
Restricted
Lane changes more difficult
Minor incidents will be absorbed, but will cause
deterioration in service
Queues may form behind significant
Blockage
Speed chosen by the driver is substantially affected by
that of other vehicles.
Comfort and convenience has decreased perceptibility
at this level.

LOS D
 Speeds begin to decline with
increasing flow.
Severe restriction to driver’s ability
to manoeuvre
Drivers experience physical and
psychological discomfort
 Even minor incidents cause queuing,
traffic stream cannot absorb
disruptions

LOS E
 Represent the level at which the capacity
of the highway has been reached.
 Traffic flow conditions are best
described as unstable
Operations are volatile, virtually no usable
gaps
Drivers levels of comfort are very poor
 Even minor incidents causing extensive
queuing and breakdown

LOS F
 This describes the state of
breakdown or forced flow with
flows exceeding capacity.
 Operating conditions are highly
unstable
Constant queuing and traffic moving
on a ‘’Stop-Go” basis
Drivers comfort level is extensively
poor

LOS
 Operating conditions can be expressed
graphically with reference to basic speed
flow relationship.
 At LOS A, speed is near its max value,
restricted only by road geometry and low
flows.
 At LOS D, flows are maximised with speed
at 50% of its Max value.
 LOS F, denotes the “breakdown condition
at which both speeds and flow tend
towards zero.

DEFINITION OF TERMS
 To determine a road’s LOS, a comprehension of the relationship
between hourly volumes, peak-hour factor and service flow is vital.
Hourly-volume (V) = The highest volume within a 24-h period.
Peak-hour factor (PHF)= The ratio of the hourly volume to the peak
15-min flow (V15) enlarged to an hourly value
PHF = V/ V15 * 4
Service flow (SF) = The peak 15-min flow (V15) enlarged to an
hourly value . SF = V15 * 4

DESIGN OF LEVEL OF SERVICE
 This is the desired quality of traffic condition from a driver’s
perspective (used to determine number of lanes)
Design LOS is higher for higher functional classes
Design LOS is higher for rural areas
LOS is higher for level/rolling than mountainous terrain
Other factors include: adjacent land use type and development
intensity, environmental factors, and aesthetic and historic values
 Design all elements to same LOS (use HCM to analyze)

MAXIMUM SERVICE FLOW RATES FOR MULTILANES
 The HCM generates max flow values obtainable on multilane
highways, given a certain speed limit and prevailing LOS
The values assume that ideal conditions exist (i.e
a. all carriageways have a standard 3.65m,
b. there are no obstructions within 1.83m of the edge,
c. there are no heavy good vehicles, buses or recreational vehs on
the road,
d. driver population consists of regular weekday drivers
e. The road is divided by physical barrier
f. Roads are rural based
g. No pedestrians
h. No on-street parking

MAXIMUM SERVICE FLOW RATES FOR MULTILANES
 In an Ideal conditions, Max service flow,
Where
N= is the number of lanes in each direction,
Cj = is the capacity of a standard highway lane for a given design
speed j. TABLE 1

SOLUTION
 Under the prevailing ideal conditions, therefore, with reference
to table 2, the ratio of flow to capacity is greater than 0.54 but
less than 0.7. The highway thus provides level of service C.

 In a non- Ideal conditions, Max service flow,
 When lane widths are narrower than 3.65
 Lighting post or any such obstruction are closer than
1.83 from the edge of the travelled pavement
pavement, an adjustment factor fw must be introduced.

HIGHWAY CAPACITY STUDIES. Lecture Notes in Civil Engineering

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