![]() (viii) The results may be summarised in the form of a timing diagram or a phasing diagram. (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. (v) Providing suitable amber times, the times of red, green and amber aspects are obtained. (iv) The optimum signal cycle, C 0, is given by In the case of mixed traffic, all the q- and S-values are to be obtained after converting to equivalent PCU values. (iii) The ratios y 1 = q 1/S 2, y 2 = q 2/S 2, … are calculated. ![]() (ii) The normal flows q 1, q 2, … on the approach roads, during the design peak hour traffic are also obtained from field observations In the absence of data, the values are determined using RRL recommendations already given under the definition for ‘saturation flow’. (i) The saturation flow of traffic, S 1, S 2,… 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. This is considered to be a rational approach as it has a mathematical basis. 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. (vi) This iterative procedure is repeated until there is good agreement between the calculated and assumed values of the cycle lengths.įinally, the timing and phasing diagrams are shown to summarise the design. Otherwise, a different trial cycle, C 2 (if C 1 < C 1, C 2 < C 1) is assumed, and steps (i) to (iv) are repeated. (v) If C’ 1 is very nearly equal to the assumed values C 1, it is taken to be the design cycle. (iv) The cycle length C’ 1, equals to (G 1 + G 2 + A 1 + A 2) seconds (iii) The amber periods A 1 and A 2 are calculated based on the approach speeds (or assumed to range from 2 to 5s) The number of cycles in the 15-minute period, n 1 will then be 900/C 1.Īssuming the time headway as 2.5 seconds, the green periods for roads 1 and 2 clear the during the trial cycle are got as – (ii) A trial cycle of C seconds is assumed. (i) The 15-minute traffic counts n 1 and n 2 on the two roads 1 and 2 are observed during peak hour of flow. ![]() The following are the steps in this method: The following methods are commonly used for the design of traffic signals: Methods for the Design of Traffic Signals: For smaller widths, reduced values of S are recommended. 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. S = 525 w passenger car units (PCU) per hour … (4.43) 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.įor new signal installations, the RRL recommends the use of the following formula for determining saturation flow – This indicates the number of vehicles passing an intersection with minimum headway during the whole of a ‘green’ period. 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. The more the queue length the less is the level of service. This indicates the number of waiting vehicles accumulating near the intersection during a signal cycle. A typical phasing diagram for a simple rectangular intersection is shown in Fig. ![]() This diagram indicates the movements of traffic/pedestrians in the respective phases of the signal system. A typical timing diagram for a simple two-phase traffic system is shown in Fig. 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. 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.
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