Signal control of junctions is one of the effective means to control conflicting traffic at junctions. Traffic signals work on the basis of allocating separate time periods to conflicting traffic movements at a highway intersection so that the available carriageway space is utilised as efficiently and safely as possible. Priority of movement of the traffic can be varied with time through the cycle of the signals. Within urban areas in particular, in situations where a road has a number of intersections along its entire length, signal linking, which is an aspect of the operations of the signal-controlled junctions can be used as a method for allowing almost continuous progression of traffic through the route.
Before a decision to use traffic control signal lights is made, it is necessary to analyse and compare the cost of installation, operation, and maintenance of these signals against the economic losses associated with delays when they are not present, as well as the reduction of accidents which they usually offer.
Mathematically, if;
(Cost of installation + operation + maintenance) < (economic gain + reduction in accidents), signal-controlled junctions should be used.
Different colours are used for signal control and these vary from country to country. The common colours in use are GREEN (which permits vehicles on the lane it faces to move), RED (which requires the vehicles on the lane it faces to wait), and finally AMBER which is a transition colour from red to green or green to red.
Design of Traffic Signal Control
The basis for the design of a signal-controlled junction is the ratio of demand to capacity for each flow path. The capacity of a given flow path is expressed as its saturation flow which is determined by summing the saturation flows of the individual lanes within each of the pathways. Saturation flow is defined as the maximum traffic flow capable of crossing the stop line assuming 100% green time. The design of traffic signal control can be carried out in the following steps:
Step 1: Estimation of Capacity
To estimate the capacity, information on the number of lanes, width of lanes, location of lanes, weather conditions, gradient, and turning movements are required.
Capacity is the sum of the saturation flow of all lanes within an approach road. This is usually measured in pcu/hr. The number of lanes, width of lanes, and location of lanes affect this value. For an average lane width of 3.25 m, and assuming no gradient, the saturation flow is 1940 pcu/hr for nearside lanes and 2080 pcu/hr for non-nearside lanes.
The weather conditions are necessary to be considered because during wet weather conditions, flows decrease to 6% below their dry weather values.
From the gradient point of view, it is noted that for every 1% increase in uphill gradient (measured over a 60 m distance back from the stop-line), the saturation flow value will decrease by 2%. Downhill gradients do not affect saturation values.
With regard to turning movement, where the turning traffic is unopposed, saturation flows will decrease as the proportion of turning traffic increases. Where turning traffic is opposed, the saturation flow will depend on the number of gaps in the opposing traffic flow together with the amount of storage space available to those vehicles making this traffic movement.
Saturation flows can be determined by the following equations proposed by Kimber et al. (1986):
For Unopposed Traffic Streams,
The saturation flow is given by:
S1 = (So – 140dn)/ (1 + 1.5 f/r) pcu/hr
Where,
So = 2080 – 42dg x G + 100 (w – 3.25)
dn = 1 (nearside lane) or 0 (non-nearside lane)
f = proportion of turning vehicles in the lane under scrutiny
r = radius of curvature of vehicle path, metres
dg = 1 (uphill entry roads) or 0 (downhill entry roads)
G = percentage gradient of the entry road
w = entry road lane width, metres.
For Opposed Traffic Streams,
The saturation flow in a given lane for right-turning opposed streams is given by:
S2 = Sg + Sc (pcu/hr)
The first term, Sg is the saturation flow occurring during the ‘effective green’ time within the lane of opposed mixed turning traffic.
Mathematically,
Sg = (So – 230)/ (1 + (T – 1)f )
T = 1 + 1.5/r + t1/t2
t1 = 12Xo2 / (1 + 0.6 (1 – f ) Ns)
Xo = qo/λn1so
t2 = 1 – (fXo)2
Xo is the degree of saturation on the opposing entry arm (ratio of flow to capacity).
qo is the actual flow on the opposing arm, measured in vehicles per hour of green time (excluding non-hooking right-turning vehicles).
λ is the effective green time divided by the total cycle time and C.
n1 is the number of lanes within the opposing entry arm.
so is the saturation flow for each of the lanes on the opposing entry arm.
Ns is the number of storage spaces within the junction that the right-turning vehicles can use so as not to block the straight-ahead stream.
The second term, Sc is 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).
Mathematically,
Sc = P (1 + Ns) (fXo)0.2 x 3600/λC
P = 1 + Σi (αi – 1)pi
P is the conversion factor from vehicles to passenger car units.
αi is the pcu value of vehicle type i.
pi is the proportion of vehicles of type i.
The passenger car unit values used in connection with the design of signal-controlled junctions are given below:
Table 1: Vehicle type versus PCU equivalent
| Vehicle Type | PCU Equivalent |
| Car/light vehicle | 1.0 |
| Medium commercial vehicle | 1.5 |
| Heavy commercial vehicle | 2.3 |
| Bus/coach | 2.0 |
Step 2: Estimation of Effective Green Time
Effective green time is defined as the length of time that would be required to get a given discharge rate over the stop line if the flow commenced and finished simultaneously and instantaneously on the change of colour as displayed on the signal head. This value is estimated based on the analysis of the flow of vehicles across the stop line at an intersection.
The discharge of vehicles across the stop line starts at the beginning of the green period and finishes at the end of the amber period. The intervals of time between the start of actual green time and the start of effective green and between the end of effective green time and the end of the amber period are termed lost time.
At the start of any given cycle, when the light goes green and traffic begins to move off, the flow across the stop line rises from zero, gradually increasing until saturation flow is achieved. The flow level remains steady until the light turns amber at the end of the phase. Some vehicles will stop; others may take some time to do so. The flow returns to zero as the lights turn red. It can be seen from Figure 1 that the actual green time plus the amber period is equal to the effective green time plus the two periods of lost time at the beginning and end of the cycle.
The effective green time is thus the length of time during which saturation flow would have to be sustained in order to obtain the same quantity of traffic through the lights as is achieved during an actual green period. It is denoted by a rectangle in Figure 1. This rectangle has exactly the same area as that under the actual flow curve. Normally, the lost time is assumed to be taken as equal to 2 seconds, with the amber time set at 3 seconds. Effective green time is thus equal to actual green time plus 1 second.
Step 3: Estimation of Lost Time
Lost time per cycle consists of the time lost during the green period (generally taken as 2 seconds per phase) plus the time lost during what is known as the intergreen period. The intergreen period is defined as the period between one phase losing the right of way and the next phase gaining the right of way, or the time between the end of green on one phase and the start of green on the next.
The intergreen period provides a suitable time during which vehicles making right turns can complete their manoeuvre safely having waited in the middle of the intersection. If the amber time during the intergreen period is 3 seconds and the total intergreen period is 5 seconds, this gives a lost time of 2 seconds, as this is the period of time for which all lights show red or red/amber, a time during which no vehicle movement is permitted. The period of time lost to traffic flow is referred to as lost time during the intergreen period. It should not be confused with lost time due to starting delays at the commencement of each phase.
Step 4: Estimation of Optimum Cycle Time
When there is a change of signal in the operation of a signal-controlled junction, some time is lost which is denoted as lost time. The lost time is always part of cycle time. The lost time is often a fixed value. Thus, if the cycle time is short, the lost time takes a large junk of the cycle time which would lead to lengthy delays but if the cycle time is long, the lost time is an insignificant part of it, thus, making the green not exhaustive. In order to ensure the most effective operation of the junction through the discharge of traffic on the waiting queue in the approach road, an optimum cycle time is required. This can be estimated by an expression developed by Webstar in 1958 to minimise the total delay to all streams on the approach roads.
Optimum cycle time, Co = (1.5L + 5) / (1 – Y)
Where,
L = total lost time per cycle
Y = the sum of the maximum y values for all of the phases that make up the cycle (y is the ratio of actual flow to saturation flow on each approach).
Typical values of cycle time range from 30 seconds to 90 seconds. Minimum and maximum values of 25 seconds and 120 seconds respectively are recommended.
Step 5: Estimation of Average Vehicle Delay
The average delay per vehicle at the signal-controlled junction can be estimated by the expression given by Webster (1958). In the method,
Where,
d = average delay per vehicle
c = cycle length
λ = effective green time divided by cycle time
q = flow
s = saturation flow
x = q/λs
Where,
C = correction term, which can be taken as 10% of the sum of the first two terms
Thus, 1 – 10% = 1 – 0.1 = 0.9
Thus, the delay can be re-written as,
Step 6: Estimation of Queue Length
Estimation of queue length is important at the beginning of the green period because that is when the maximum queue length is usually obtained.
When there is an Unsaturated Approach,
Queue length, Nu = qr
Where,
Nu = queue length at the commencement of the green period (assuming the approach is unsaturated)
q = actual flow rate
r = effective red period (cycle time – effective green time)
When there is a Saturated Approach,
Where,
Ns = queue length at the commencement of the green period (assuming the approach is saturated)
d = average delay per vehicle on the approach
The greater of the two values between Nu and Ns is taken as the queue length.
Operation of Signal-Controlled Junction
Step 1: Phasing
Phasing is a process of separating conflicting traffic. Depending on the number of conflicts, there can be a one-phase system, two-phase system, three-phase system etc.
Step 2: Stage Control
This is a means to control traffic movement at signal-controlled junctions. It is a predetermined sequential step used to vary the control movements at intersections. A stage usually commences from the start of an amber period and ends at the start of the following stage.
Step 3: Signal Linkage
This is necessary in urban settings where signal-controlled junctions are closely spaced. This signal linkage or coordination can be achieved by the use of a time–and–distance diagram. The time and distance diagram can be developed by following the processes below:
- Determine the cycle time of all intersections and choose the maximum cycle time as the cycle time of the entire network, C1.
- Determine the lost time of the key intersection which is the intersection with the maximum cycle time.
- Determine the effective green time and actual green time of the key intersection which in essence would determine the minimum actual green time along this axis at the other intersections.
- Determine the minimum and maximum actual green time for the minor intersections. The maximum actual green time in each case for the non-key intersections can be derived through the determination of the smallest acceptable green time for the minor road phases. The minimum actual green time for the non-key intersections can then be calculated by the addition of the lost time per phase and subtraction of amber time.
Minimum effective greennon-key intersectio = (yside x C1)/ 0.9
- Determine the distance between the individual intersections and assume an average speed of progression in both directions along the major axis.
- Plot the time-and-distance diagram.
Advantages of Signal Control Lights
Signal control lights are useful in the following ways:
- Reduction in delay to motorists and pedestrians moving through the junction.
- Reduction of accidents at the junction.
- Improvement in the control of traffic flow into and through the junction in particular and the area in general, thereby minimising journey times.
- Chosen traffic management policies can easily be implemented.
Disadvantages of Signal Control Lights
The negative impacts of signal control lights include:
- They must undergo frequent maintenance along with frequent monitoring to ensure their maximum effectiveness.
- There can be inefficiencies during off-peak times leading to increases in delay and disruption during these periods.
- Increases in rear-end collisions can result.
- Signal breakdown due to mechanical/electrical failure can cause serious interruptions in traffic flow.
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References
Rogers, M. (2003). Highway Engineering. Blackwell Publishing Limited, UK
