Traffic Signal
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Traffic Signal is define as any power operated traffic control device or a sign by which traffic is warned or directed to take some special action.Traffic signals is a device which is used to direct the traffic to stop and proceed at intersections using red and green light signals automatically.
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| Traffic Signals |
Advantages of Traffic Signals
(i) They provide orderly movement of traffic and increase the traffic handing capacity of most of the intersections at grade.
(ii) They reduce certain types of accident mainly the right angled collisions.
(iii) In the effective use of signal system, reasonable speed is maintained along the major traffic.
(iv) Signals provide a chance to crossing traffic of minor road to cross path of continuous flow of traffic stream at reasonable intervals of time.
(v) Automatic traffic signal may workout to be economical when compared to manual control.
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| Traffic Signals |
Disadvantages of Traffic Signal
(i) It has some disadvantages.Improper design is one of them. Signals may load to violations of the control system.
(ii) It can help to failure of signal due to electric power failure or any other defect may cause confusion to the road users.
Types of Traffic Signals
(a) Traffic Control Signals-
Traffic control signals have three coloured light glows facing each direction of traffic flow.The red light is meant for "STOP", Green light indicates "GO", and the amber light allows the clearance time for the vehicles which enter the intersection area by the end of green time.Traffic control signals are of three types-
(i) Fixed time signal.
(ii) Manually operated signal.
(iii) Traffic actuated system.
In Fixed time signal timing of each phase of the cycle is predetermined base on the traffic studies.The main drawback of this is that some traffic on one road may be almost nil and traffic on cross road may be quite heavy but signal operates with fixed timing.Traffic actuated signals are those in which the timings of phase and cycle are changed according to traffic demand.
(b) Pedestrian Signal-
It is used to give the right of way to pedestrians to cross the road when the vehicular traffic shall be stopped by stop signal.
(c) Special Traffic Signal-
Special Traffic Signal such as "FLASHING BEACONS" are meant to warn the traffic. When signal is flashing red then the vehicles shall stop before entering the nearest crosswalk at an intersection.
Types of Traffic Signal System
There are four types of traffic signals-
(i) Simultaneous System- In this system all the signals show the same indication at the same time. For the division of cycle is also the same at all intersections, this system does not work satisfactorily.
(ii) Alternate System- It shows opposite indication in a route at the same time and this system is more satisfactorily.
(iii) Simple Progressive System- A time schedule is made to permit, as nearly as possible a continuous operation of groups of vehicles along the main road at reasonable speed.
(iv) Flexible Progressive System- In this system we can vary the length of the cycle, cycle division and the time schedule at each signalized intersection automatically with the help of computer.
Elements used in the Design of Traffic Signals
(a) Cycle Time-
It indicates the time interval between starting of signal such as green to the next time green signal.
(b) Interval-
It indicates the change from one stage to another.
(i) Change Interval- It also called yellow time and it indicates the interval between green and red signal.
(ii) Clearance Interval- It also known as all red time and included after each yellow interval indicating a period during all signal phases shows red and it is used for clearing of vehicles at the intersection.
Total Cycle Time = Green Interval + Red Interval + Change Interval.
(c) Phase-
A phase is taken as the sum of green interval and clearance interval.During green interval. During green interval, non conflicting movement are assigned into each phase.
(d) Lost Time- It represents the time during which the intersection is not effectively utilised for any movement.
(e) Effective Green Time(gi)-
Effective Green Time is the actual time available for vehicle to cross the intersection.
gi = Gi + Ai + t
where, Gi = Actual green time, Ai = Amber time, t = Lost time.
Traffic Signals
(f) Lane Capacity-
If every vehicle requires 'h' seconds time headway to cross the kerb line and assure the signal is always green then saturation capacity.
Saturation Capacity, S = 3600/h veh/ hr/ lane
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