Highway Engineering

The relationships between trips generated and the relevant variables are

expressed as mathematical equations, generally in a linear form. For example,

the model could take the following form:

(2.1)

where

Tij=number of vehicle trips per time period for trip type i(work, non-work)

made by household j

Z =characteristic value nfor household j, based on factors such as the house-

hold income level and number of cars available within it

a=regression coefficient estimated from travel survey data relating to n

A typical equation obtained for a transportation study in the UK might be:

where

T =total number of trips per household per 24 hours

Z 1 =family size

Z 2 =total income of household

Z 3 =cars per household

Z 4 =housing density

TZZZZ=+^0007 .^1234 +^0. ^005 +0 95.-0 003.

TZZ Zij=+011 22j+ j++aaL n nj a a

20 Highway Engineering

Example 2.1 – Basic calculation of trip rates

The following model is compiled for shopping trips generated during the

weekly peak hour for this activity (5.30to 6.30on Fridays). The rela-

tionship is expressed as follows:

where

T =total number of vehicle-based shopping trips per household in peak hour

Z 1 =household size

Z 2 =annual income of household (in £000s)

Z 3 =employment in neighbourhood (in 00s)

Calculate the trip rate for a household of four people with an annual income

of £30 000 within a neighbourhood where 1000 people are employed.

Solution

(The negative sign in the above equation arises from the reduced likelihood

of a non-work related trip occurring within an area of high employment.)

Number of trips

vehicle trips

=+ + -

=

015 01 4 00130 0145 10

23

... .*

.

TZZZshopping=+015 01..* .* .*12 3+^001 - 0 145