Highway Engineering

34 Highway Engineering

Example 2.6 Contd

These probabilities can also be calculated using Equation 2.16:

During the morning peak hour, 910 commuters will therefore travel by car

with the remaining 90 taking the bus.

After construction of the new bus lane, the changed journey times alter

the utilities as follows:

Based on these revised figures, the new modal splits are:

Post construction of the bus lane, during the morning peak hour, 800 (-110)

commuters will now travel by car with 200 (+110) taking the bus.

Thus, the introduction of the bus lane has more than doubled the number

of commuters travelling by bus.

Peee

Peee

CAR

BUS

=+()= ()

=+()= ()

• --

07 07 07

07 07 07

080 80

020 20

...

...

.%

.%

U

U

C

B

=-()- ()=

=-()- ()=-

24 02 4 00330 0 70

00 0205 00320 070

....

.....

Pe

Pe

CAR

BUS

=+()()= ()

=+()()= ()

()--

()--()

11 09191

11 0099

13 10

10 13

..

..

.%

.%

Peee

Peee

CAR

BUS

=+()= ()

=+()= ()

• --

10 10 13

13 10 13

091 91

009 9

...

...

.%

.%

2.7 Traffic assignment,

Tr affic assignment constitutes the final step in the sequential approach to traffic

forecasting. The output from this step in the process will be the assignment of

precise quantities of traffic flow to specific routes within each of the zones.

Assignment requires the construction of a mathematical relationship linking

travel time to traffic flow along the route in question. The simplest approach

involves the assumption of a linear relationship between travel time and speed

on the assumption that free-flow conditions exist, i.e. the conditions a trip maker

would experience if no other vehicles were present to hinder travel speed. In this

situation, travel time can be assumed to be independent of the volume of traffic

using the route. (The ‘free-flow’ speed used assumes that vehicles travel along

the route at the designated speed limit.) A more complex parabolic speed/flow

relationship involves travel time increasing more quickly as traffic flow reaches

capacity. In this situation, travel time is volume dependent.

In order to develop a model for route choice, the following assumptions must

be made: