Highway Engineering

Finally, since DHV is a two-directional flow (as is AADT), the flow in the
peak direction (the directional design hour volume (DDHV)) is estimated by
multiplying it by a directional factor D:

DDHV=¥¥KDAADT (4.24)

Basic Elements of Highway Traffic Analysis 91

Example 4.6

A divided rural multi-lane highway is required to cope with an AADT of

40 000 vehicles per day.

A 70 mph design speed is chosen with lanes a standard 3.65 m wide and

there are no obstructions within 1.83 m of any travelled edge. The traffic is

assumed to be composed entirely of private cars and the driver population

is ideal.

The peak hour factor is 0.9 and the directional factor,D, is estimated at

0.6. The highway is required to maintain level of service C. It is to be designed

to cope with the thirtieth highest hourly volume during the year.

Calculate the required physical extent of the highway, i.e. the number of

lanes required in each direction.

Solution

We can now calculate the service flow, knowing the hourly volume and the

peak hour factor, as follows:

In order to calculate the required number of lanes, Equation 4.18 can be

rearranged as follows:

(4.25)

Assuming the following:

Cj =2000 (design speed =70 mph)

fw =1.0 (all lanes standard, no obstructions)

fHV=1.0 (no trucks, buses or recreational vehicles in traffic stream)

fp =1.0 (ideal driver population)

fE =1.0 (rural divided multi-lane highway).

NC

v

c

ij ff ff

i

=∏¥ whvpE

Ê

Ë

ˆ

̄ ¥¥ ¥¥

Ê

Ë

ˆ

SF() ̄

SF PHF

= 2880

vh

=∏

∏

=

V

09

3200

.

DDHV AADT

= 0.12

vh

=¥¥

¥¥

=

KD

06 40 000

2880

.

Contd