Highway Engineering

X 0 is the degree of saturation on the opposing entry arm (ratio of flow to

capacity)

q 0 is the actual flow on the opposing arm, measured in vehicles per hour of green

time (excluding non-hooking right-turning vehicles)

lis the effective green time divided by the total cycle time, C

n 1 is the number of lanes within the opposing entry arm

s 0 is the saturation flow for each of the lanes on the opposing entry arm

Nsis the number of storage spaces within the junction which the right-turning

vehicles can use so as not to block the straight ahead stream.

(2) Sc

Scis the saturation flow occurring after the ‘effective green’ time within the lane

of opposed mixed turning traffic.

(During a traffic phase, the effective green time is the actual green time plus the

amber time but minus a deduction for starting delays.)

Sc=P(1 +Ns) (fX 0 )0.2¥3600/lC (5.32)

P= 1 +Si(ai -1)pi (5.33)

Pis the conversion factor from vehicles to passenger car units

aiis the pcu value of vehicle type i

piis the proportion of vehicles of type i.

The passenger car unit values used in connection with the design of a signalised

junction are given in Table 5.6.

136 Highway Engineering

Vehicle type pcu equivalent

Car/light vehicle 1.0

Medium commercial vehicle 1.5

Heavy commercial vehicle 2.3

Bus/coach 2.0

Table 5.6pcu values

Example 5.5 – Calculation of saturation flow for both opposed and

unopposed traffic lanes

The approach road shown in Fig. 5.30 is composed of two lanes, both

3.25 m wide. The nearside lane is for both left-turning and straight-ahead

traffic, with a ratio of 1 : 4 in favour of the straight through movement. The

non-nearside lane is for right-turning traffic. This movement is opposed. The

degree of saturation of the opposing traffic from the north is 0.6.

Contd