Design traffic For Highways

Civil Engineering

1. Design Traffic Volume

2. Types:
Uninterrupted flow facilities:
-These facilities are those on which no external factors
cause periodic interruption to the traffic stream.
-Example: freeways, limited-access facilities, where
there are no traffic signal, stop or yield signs, or
surface intersections.
-It may also exist in long sections of rural highway
between signalized intersections
Interrupted flow facilities:
- These facilities have external devices that periodically
interrupt traffic flow (the principal device creating
interrupted flow is the traffic signal)

3. Traffic volume is defined as the number of vehicles that
pass a point on a highway, or a given lane or direction of
a highway, during a specified time interval.
A. Daily volumes:
- Average Annual Daily Traffic: (AADT):
- Average Annual Weekday Traffic (AAWT):
- Average Daily Traffic (ADT):
- Average Weekday Traffic (AWT):
Note: The unit is vehicles per day (vpd).
Daily volumes are used to establish trends over time and
for planning purposes.
Daily volumes generally are not differentiated by
direction or lane but are totals for an entire facility at the
specified location.

4. -Average Annual Daily Traffic (AADT): is the average 24
hour traffic volume at a given location over a full
365-day year
-– that is the total number of vehicles passing the site in a year
divided by 365
-Average Annual Weekday Traffic (AAWT): is the average
24-hour traffic volume occurring on weekdays over a
full year.
-AAWT is computed by dividing the total weekday traffic
volume for the year by 260.
-This volume is of considerable interest where weekend
traffic is light, so that averaging higher weekday volumes
over 365 days would mask the impact of weekday traffic.

5. -Average Daily Traffic (ADT): is an average 24-hour traffic
volume at a given location for some period of time
less than a year.
-While an AADT is for a full year, an ADT may be measured for
six months, a season, a month, a week, or as little as
two day. an ADT is a valid number only for the period over
which it was measure.
-Average Weekday Traffic (AWT): is an average 24-hour
traffic volume occurring on weekdays for some period
of time less than one year, such as for a month or a
season. The relationship between AAWT and AWT is analogous
to that between AADT and ADT

6. AADT and AAWT are used for several transportation analyses:
◦Computation of accident rates in terms of 100 million vehicles miles
◦Establishment of traffic volume trends
◦Evaluation of the economic feasibility of highway projects
◦Development of freeway and major arterial street systems
◦Development of improvement and maintenance programs
-ADT and AWT are used for several transportation analyses:
-Measurement of current demand
-Evaluation of existing traffic flow

8. B. Hourly Volumes:
◦Season Change ® Daily Volumes ® Planning
◦Hourly ® Design and operational analysis
◦Peak-Hour: The single hour of the day that has the highest
volume.
◦This volume is not a constant value from day to day, season to
season
The traffic volume within the peak hour is of greatest interest to
traffic engineers in design or operational analysis.
Note: - The peak-hour volume is generally a directional volume that
is, a volume in which the directions of flow are separated.

9. 9
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Highway Capacity
Highly Congested
Source: www.ecn.purdue.edu/~darcy

10. - Highways must be designed to adequately serve the peak-
hour traffic volume in the peak direction of flow.
- Most operational analysis must address conditions existing
during periods of peak traffic volume.
Peak hour volumes are sometimes estimated from
projections of the AADT. It is referred to as the
“directional design hour volume” (DDHV)
DDHV = AADT * K * D (* T)
Where:
K – proportion of daily traffic occurring during the peak hour
D – proportion of peak hour traffic traveling in the peak direction of
flow

11. Used for several transportation analyses:
◦Functional classification of roads
◦Design of geometric characteristics of highways (number of
lanes)
◦Capacity analysis
◦Development of programs related to traffic operations
◦Development of parking regulations

12. C. Sub hourly volumes and Rates of Flow
Flow (q) is the equivalent hourly rate at which vehicles pass
a point on a highway during a time period less than 1
hour.
Minimum Time Interval = 15 minutes or 900 seconds
(2000 Highway Capacity Manual)
The quality of traffic flow is often related to short-term
fluctuations in traffic demand
It is often necessary to design facilities and analyze
traffic conditions for a period of maximum rate of
flow within the peak hour
veh/h
n timeobservatio
nobservatio during vehiclesofnumber
=qflow of rate =

13. Example of volumes and rate of flow
Time Volume Rate of flow
interval(vehicles) (vehicles/h)
5:00-5:15 PM 950 950*4=3800
5:15-5:30 PM 11501150*4=4600
5:30-5:45 PM 12501250*4=5000
5:45-6:00 PM 10001000*4=4000
For the hour
5:00-6:00 PM 4350 (veh/h)
A facility may have capacity adequate to serve the peak-hour
demand, but short-term peaks of flow within the peak hour
may exceed capacity, thereby creating a breakdown.

14. Peak-Hour Factor (PHF): is the ratio of the volume
occurring during the peak hour to a maximum rate of
flow during a given time period within the peak hour
Usually, 15 minute periods of flow are used (2000
Highway Capacity Manual)
Where,
HV – Hourly Volume
V
15
– Maximum 15 minute volume within the hour
flow of rate maximum
umehourly vol
= PHF
15
V4
HV
15
V(60/15)
HV
= PHF
*
=
*

15. Example of PHF:
HV= 4350 vehicles
V
15 = 1250 vehicles
= 0.87
NOTE:
0.25 £ PHF £ 1.00, normal between 0.70 and 0.98
Lower PHF indicates a greater degree of variation in flow
during the peak-hour.
15
V4
HV
= PHF
*
01254
4350
= PHF
*

16. 16

17. 17
DHV = Peak-Hour Volume
PHF
Example
Peak hour volume from previous = 375 vph
PHF = 0.625
DHV = 375 = 600 vph
0.625
Note: the traffic you design for is the busiest 15
minutes during the peak hour … another way to
think of it is 150 vehicles per 15 minutes = 600
vehicles per 60 minutes