Highway Engineering

148 Highway Engineering

Example 5.9 Contd

A1

A2

A3

400

metres

400

metres

N

Figure 5.33

Diagrammatic layout

of network of urban-

based intersections.

Junction A1:

Co=[(1.5 ¥8)+5] ∏[1 -(0.375 +0.444)]

=94 seconds

Junction A2:

Co=[(1.5 ¥8)+5] ∏[1 -(0.363 +0.474)]

=104 seconds

Junction A3:

Co=[(1.5 ¥8)+5] ∏[1 -(0.313 +0.444)]

=70 seconds

From the above figures it can be seen that the longest of the three cycle times

is for junction A2 – 104 seconds. This is the key junction and is thus adopted

for the entire network.

Minimum actual green time:

Assuming a cycle time of 104 seconds, the effective green time can be esti-

mated along the main north/south axis by use of theycritvalues at junction

A2. Of the 104 seconds, 8 seconds is lost time, therefore the combined effec-

tive green time in both directions is 96 seconds (104 -8). The relative values

of the critical ratios are then used to estimate the proportion of this green

time allocated to the north/south and east/west directions.

Since

ycrit(N/S) =0.363

ycrit(E/W) =0.474

Contd