Traffic Signals: Notes, Importance and Methods Used for the design of traffic signals. The following definitions, concepts and explanations are useful or important in and relevant to the design of traffic signals.
(i) Time Cycle:
The period of time required for the complete sequence of signal aspect at any one arm of an intersection from red to the onset of the next red is known as the ‘cycle time’. This varies from 35 seconds to 60 seconds for a simple right-angled intersection depending upon the width of road and traffic volume.
The part of the signal cycle allocated any traffic movement or a combination of traffic movements.
Any part of the signal cycle during which signal indications do not change.
(iv) Lost Time:
Time lost at the beginning and termination of green phases due to starting time and termination time. As soon as the signal turns green, the rate of discharge (or vehicle movement) starts to pick up and some time is lost before the flow reaches the maximum value. Similarly at the termination of the green phase, the flow tends to taper off, involving a further loss of time.
The lost time for the phase is total green time and amber time for the phase, less effective time.
(This will be better appreciated after the definition of ‘saturation flow’ is understood)
(v) Amber Period:
The transition interval between the termination of a related green signal and exhibition of a red signal is called the ‘clearance amber’ as this helps the traffic at the intersection to clear before green is flashed for traffic from the perpendicular or other oncoming directions.
Amber signal is again flashed between the termination of the red signal and the start of the related green movement; this is called the ‘initial amber’ or ‘get ready’ indication for the traffic in a particular direction. If this amber overlaps the last part of the red indication, it is called ‘red-amber’.
In Indian practice, the amber period is generally 2 seconds, but may last up to 4 seconds, depending upon the traffic and pedestrian movement. For higher speeds, the amber period for clearance will be more.
(vi) Inter-Green Period:
This is the time from the end of the green period of the phase losing right of way to the beginning of the green period of the phase gaining right of way.
(vii) Timing Diagram:
This is a pictorial representation of the time cycles of the phases, on which the times of the signal indications – red, amber and green are marked in the correct sequence. A typical timing diagram for a simple two-phase traffic system is shown in Fig. 4.45.
(viii) Phasing Diagram:
This diagram indicates the movements of traffic/pedestrians in the respective phases of the signal system. A typical phasing diagram for a simple rectangular intersection is shown in Fig. 4.46.
(ix) Queue Length:
This indicates the number of waiting vehicles accumulating near the intersection during a signal cycle. The more the queue length the less is the level of service.
(x) Level of Service:
This is a measure of a number of factors in traffic operation and is measured by the vehicle delay, the queue length and the probability of a vehicle entering the intersection during the first phase on its arrival.
(xi) Saturation Flow (S):
This indicates the number of vehicles passing an intersection with minimum headway during the whole of a ‘green’ period. The method of measuring saturation flow is given in “A method of measuring saturation flow at traffic signals, Road Note No.34”, Road Research Laboratory, HMSD, London, 1963.
For new signal installations, the RRL recommends the use of the following formula for determining saturation flow –
S = 525 w passenger car units (PCU) per hour … (4.43)
S = Saturation flow
w = Width of approach road in metros.
This works out to approximately 160 PCU for every 0.3 m width of the road and is applicable for w ranging from 5.50m to 18m. For smaller widths, reduced values of S are recommended.
[Note- Corrections to the saturation flow are needed when the approach road is on a gradient, as also under opposing flow and right turning traffic conditions. The details of these corrections are out of scope of the present treatment]
Methods for the Design of Traffic Signals:
The following methods are commonly used for the design of traffic signals:
(a) Trial cycle method
(b) Webster’s minimum delay method
(c) IRC method
(a) Trial Cycle Method:
The following are the steps in this method:
(i) The 15-minute traffic counts n1 and n2 on the two roads 1 and 2 are observed during peak hour of flow.
(ii) A trial cycle of C seconds is assumed. The number of cycles in the 15-minute period, n1 will then be 900/C1.
Assuming the time headway as 2.5 seconds, the green periods for roads 1 and 2 clear the during the trial cycle are got as –
(iii) The amber periods A1 and A2 are calculated based on the approach speeds (or assumed to range from 2 to 5s)
(iv) The cycle length C’1, equals to (G1 + G2 + A1 + A2) seconds
(v) If C’1 is very nearly equal to the assumed values C1, it is taken to be the design cycle. Otherwise, a different trial cycle, C2 (if C1 < C1, C2 < C1) is assumed, and steps (i) to (iv) are repeated.
(vi) This iterative procedure is repeated until there is good agreement between the calculated and assumed values of the cycle lengths.
Finally, the timing and phasing diagrams are shown to summarise the design.
(b) Webster’s Minimum Delay Method:
In this method, the optimum signal cycle corresponding to the minimum total delay to the traffic at the signalised intersection is obtained after detailed studies of cycle time and delay vis-a-vis the traffic volumes approaching the intersection. This is considered to be a rational approach as it has a mathematical basis.
The steps in the design procedure are:
(i) The saturation flow of traffic, S1, S2,… on the approach roads are obtained from field studies by noting the number of vehicles in the stream of flow during the green aspects and the corresponding time intervals.
In the absence of data, the values are determined using RRL recommendations already given under the definition for ‘saturation flow’.
(ii) The normal flows q1, q2, … on the approach roads, during the design peak hour traffic are also obtained from field observations
(iii) The ratios y1 = q1/S2, y2 = q2/S2, … are calculated.
In the case of mixed traffic, all the q- and S-values are to be obtained after converting to equivalent PCU values.
(v) Providing suitable amber times, the times of red, green and amber aspects are obtained.
(vi) If there are pedestrian signals, the ‘Walk’ and ‘Don’t Walk’ times can also be calculated based on the red aspect times on the particular roads and extra times for starting delays.
(viii) The results may be summarised in the form of a timing diagram or a phasing diagram.
(c) IRC Method:
This is basically the same as Webster’s approach, but with a few minor variations in accordance with the guidelines given by the IRC.
The following are the steps in the design:
(i) The green times required for the major and minor roads are calculated under the assumption of an initial walking delay of 7 seconds and a walking speed of 1.2m/s to cover the width of the carriageway. These are the minimum green times required for vehicular traffic on the minor and major roads in that order.
(ii) The green time required for vehicular traffic on the major road is increased in proportion to the traffic on the two roads (major/minor).
(iii) The cycle time is calculated assuming an amber time of 2 seconds each.
(iv) The minimum green time required for clearing the vehicles approaching from each road is determined, assuming that the first vehicle needs 6 seconds and all the subsequent ones need 2 seconds each (this minimum should not be less than 16 seconds).
(v) The optimum signal cycle is calculated using Webster’s formula, using the appropriate values of saturation flow for the widths of the approach roads.
(vi) The lost time is calculated from the amber time, inter-green time and the initial delay time of 4 seconds for the first vehicle, on each leg.
(vii) The signal cycle time and the phases may be revised keeping in view the requirements from steps (iv) and (v) above.