Intersection control methods; Webster signal 400/300 PCU

Methods of traffic control at an intersection

1. Uncontrolled (priority by rules): drivers give way to the vehicle on the right or on the main road; suitable for very low volume.
2. Traffic signs and markings: STOP and GIVE WAY signs, with stop lines; the minor road gives way to the major road (IRC:67, IRC:35).
3. Channelisation: islands and markings guide the movements (IRC:SP:41).
4. Rotary (roundabout): one-way circulation, no signals (IRC:65).
5. Traffic signals: fixed-time, vehicle-actuated or area-coordinated (IRC:93).
6. Police control by hand signals at peak times or special events.
7. Grade separation: flyover, underpass or interchange for very high volume.

Design of a two-phase signal by Webster's method

Data: roads A and B carry 400 and 300 PCU/h; saturation flows 1450 and 1150 PCU/h; all-red time for pedestrians $R$ = 12 s per cycle.

Step 1: Flow ratios

Flow ratio $y = q/s$ for each approach; the larger value in a phase is the critical one.

$$Y = 0.276 + 0.261 = 0.537$$

Step 2: Lost time

An amber of 2 s per phase is assumed and taken as the lost time of that phase; the all-red time $R$ = 12 s is also lost.

$$L = 2 + 2 + 12 = 16.0\ \text{s}$$

Step 3: Optimum cycle time (Webster)

$$C_0 = \frac{1.5L + 5}{1 - Y} = \frac{1.5(16.0) + 5}{1 - 0.537} = 62.6\ \text{s}$$

Adopt C = 65 s.

Step 4: Green times

Total effective green $= C - L = 65 - 16.0 = 49.0$ s, divided in proportion to $y$:

$$g_{1} = \frac{y_1}{Y}(C-L) = \frac{0.276}{0.537}(49.0) = 25.2\ \text{s}$$ $$g_{2} = \frac{y_2}{Y}(C-L) = \frac{0.261}{0.537}(49.0) = 23.8\ \text{s}$$

Because the whole amber is counted as lost time, the displayed green equals the effective green (so that $\sum G + \sum I + R = C$):

Check: 25 + 2 + 24 + 2 + 12 = 65 s.

Phase diagram

|    P1 green 25    | A2|   P2 green 24    | A2|   R 12  |