An intersection is where traffic streams want to occupy the same patch of road at the same moment. A traffic signal solves that by allocating time instead of space — each stream gets the whole junction, but only for its own share of the cycle.
This post covers what signals are, the three terms you must define precisely, the types, their advantages and drawbacks, and the four ways signals along a road can be coordinated.
What Traffic Signals Are
Traffic signals are control devices which alternately direct traffic to stop and proceed at intersections, using red and green traffic lights automatically.
Four Main Requirements
A signal must:
- Draw attention
- Provide meaning
- Provide time to respond
- Have minimum waste of time
The first three describe communication with the driver — be noticed, be understood, allow time to act. The fourth is different in kind: it concerns efficiency. Every second of red shown to an empty approach is wasted capacity, and the whole of signal design exists to minimise that waste.
Three Terms You Must Get Right
These three definitions are the foundation of every signal design calculation, and they are frequently confused.
| Term | Definition |
|---|---|
| Cycle | The period of time required for one complete sequence of signal indications |
| Phase | A part of the signal cycle allocated to a traffic movement or a combination of traffic movements |
| Interval | Any division of the signal cycle during which signal indications do not change |
How They Nest
Think of it as three levels:
Cycle — the whole repeating sequence
↳ Phase — the portion belonging to one traffic movement
↳ Interval — a stretch within which nothing changes
A phase is defined by who is being served. An interval is defined by what the lights are showing. So a phase for one road typically contains a green interval and an amber interval — two intervals, one phase.
The Three Lights
Traffic control signals have three coloured lights facing each direction of traffic flow:
- Red means stop
- Green means go
- Amber or yellow allows clearance time for vehicles that entered the intersection area by the end of green, so they can clear off
Amber is the one students describe loosely. It is not a warning that red is coming, and it is not “hurry up”. Its engineering purpose is clearance — emptying the junction of vehicles already committed to it, before the conflicting stream is released. Without it, a vehicle that entered legally on green would still be inside the junction when cross traffic starts moving.
Types of Traffic Signals
- Traffic control signals
- Fixed-time signal
- Semi-actuated signal
- Fully actuated signal
- Pedestrian signal
- Special traffic signal
Note: traffic control signals are also listed elsewhere as fixed-time, manually operated, and traffic actuated (automatic) signals. Both groupings appear in standard treatments — the second simply separates manual operation as its own category.
Fixed-Time or Pre-Timed Signals
- Set to repeat a cycle of red, amber and green regularly.
- The timing of each phase is predetermined, based on traffic studies.
- Signal timing does not change in response to changes in traffic flow.
- No vehicle detection is necessary.
The main drawback: sometimes traffic flow on one road is almost nil while the cross road is quite heavy — yet the time available for the cross road may still be less. The signal keeps showing green to an empty approach because it has no way of knowing the approach is empty.
Semi-Actuated Signals
- Timing — cycle length, green time and so on — is affected when vehicles are detected on some, but not all, approaches.
- Detection is by video, pavement-embedded inductance loop detectors, and similar means.
- Usually found where a low-volume road intersects a high-volume road — the minor and major streets respectively.
- Green time is allocated to the major street until vehicles are detected on the minor street. Green is then briefly given to the minor street, and returned to the major street.
This is a neat solution to a specific problem. The major road carries traffic almost continuously, so it should hold green by default. The minor road only occasionally has a vehicle — so it gets green only when there is actually someone waiting.
Fully Actuated Signals
- Timing is completely influenced by traffic volumes detected on all of the approaches.
- Most commonly used at intersections of two major streets, and where substantial variations exist in all approach volumes over the course of a day.
Where semi-actuated signals assume one road dominates, fully actuated signals assume none does. If every approach can be busy or quiet at different times of day, every approach needs detection.
Choosing Between Them
| Type | Detection | Suited To |
|---|---|---|
| Fixed-time | None | Steady, predictable flows |
| Semi-actuated | On some approaches | A minor road crossing a major road |
| Fully actuated | On all approaches | Two major roads with large variation through the day |
Advantages of Traffic Signals
- They provide orderly movement of traffic and increase traffic handling capacity.
- Accidents are reduced.
- Pedestrians can cross the road safely.
- Heavy traffic can flow smoothly.
- A reasonable speed of flow is maintained.
- Automatic traffic signals work without manpower.
Disadvantages of Traffic Signals
- Rear-end collisions may increase, because of the sudden stopping of vehicles.
- If the control system is not properly designed, it will cause violation.
- Confusion can occur due to signal failure — from electric power failure or malfunction of the signals.
Understanding the First Two
The rear-end collision point is important and often misread. Signals reduce accidents overall, which is advantage 2. But they change the type of accident. Right-angle collisions between crossing streams fall sharply, while rear-end collisions rise, because signals require vehicles to stop where they previously flowed. The net effect is positive — a rear-end impact is far less severe than a side impact — but the trade-off is real.
The violation point connects to signal design. If a signal makes drivers wait unreasonably long, some will jump it. Poor design does not merely waste time; it actively teaches road users to disobey, which then undermines every other signal in the network.
Coordination of Traffic Signal Systems
A single well-designed signal is of limited use if the next one along the road stops you anyway. Four general types of coordination exist for a road network:
1. Simultaneous System
- All signals along a given road show the same indication at the same time.
- The division of the cycle is also the same at all intersections.
- This system does not work satisfactorily.
The reason is easy to see. When every light turns green together, the whole platoon of vehicles starts moving — and when they all turn red together, everyone stops, regardless of where they have reached. Vehicles are forced to stop at every single signal.
2. Alternate System
- Alternate signals, or groups of signals, show opposite indications along a route at the same time.
- Operated by a single controller, by reversing the red and green connections at successive signal installations.
- Generally considered more satisfactory than the simultaneous system.
This is a genuine improvement achieved very cheaply — no new equipment, just reversed wiring. A vehicle leaving one green signal arrives at the next as it turns green.
3. Simple Progressive System
- A time schedule permits, as nearly as possible, continuous operation of groups of vehicles along the main road at a reasonable speed.
- The phases controlling “go” indications are scheduled to work at a predetermined time schedule.
- Phases and intervals at each installation may be different, but each unit works as a fixed-time signal with equal signal cycle length.
This is the “green wave”. Each signal turns green just as the platoon reaches it, so a driver travelling at the design speed meets green after green. Note the requirement of equal cycle length throughout — without it the signals would drift out of step.
4. Flexible Progressive System
- It is possible to automatically vary the length of cycle, the cycle division and the time schedule at each signalised intersection, with the help of a computer.
- This is the most efficient system of all four.
The simple progressive system fixes one green wave in advance, tuned to one set of traffic conditions. Traffic, however, changes through the day. The flexible progressive system lets a computer retune the whole network continuously — which is why it is the most efficient.
The Four Systems Ranked
| System | Key Feature | Performance |
|---|---|---|
| Simultaneous | All signals identical at all times | Does not work satisfactorily |
| Alternate | Successive signals show opposite indications | More satisfactory than simultaneous |
| Simple progressive | Fixed time schedule, equal cycle lengths | Good — provides a green wave |
| Flexible progressive | Computer varies cycle, division and schedule | Most efficient |
Quick Revision Notes
- Cycle = time for one complete sequence of signal indications.
- Phase = part of the cycle allocated to a traffic movement or combination of movements.
- Interval = a division of the cycle during which indications do not change.
- Red = stop, green = go, amber = clearance time for vehicles that entered by the end of green.
- Four requirements: draw attention, provide meaning, provide time to respond, minimum waste of time.
- Types: traffic control signals (fixed-time, semi-actuated, fully actuated), pedestrian signals, special traffic signals.
- Fixed-time needs no vehicle detection and does not respond to flow changes.
- Semi-actuated detects on some approaches — used where a minor road meets a major road.
- Fully actuated detects on all approaches — used where two major streets meet with large daily variation.
- Six advantages, three disadvantages; rear-end collisions may increase even though accidents overall reduce.
- Four coordination systems: simultaneous, alternate, simple progressive, flexible progressive (most efficient).
Mistakes Students Commonly Make
- Confusing phase and interval. A phase serves a traffic movement; an interval is a period with unchanging indications. A phase usually contains more than one interval.
- Describing amber as a warning that red is coming. Its purpose is clearance.
- Saying signals reduce all accident types. Rear-end collisions may increase.
- Mixing up semi-actuated and fully actuated. Semi detects on some approaches, fully on all.
- Claiming the simultaneous system is satisfactory. It is explicitly stated not to work well.
- Forgetting that the simple progressive system requires equal cycle length at every signal.
- Naming the simple progressive system as most efficient. That is the flexible progressive system.
Conclusion
Traffic signals share out time at an intersection rather than space, and every part of the topic follows from that idea. Cycle, phase and interval describe how the time is divided. The three signal types differ in how much they let real traffic influence that division — from none at all in fixed-time signals to complete influence in fully actuated ones. And the four coordination systems extend the same thinking from one junction to a whole road, culminating in a computer-managed network that retunes itself as traffic changes.
Frequently Asked Questions
What is a signal cycle?
The period of time required for one complete sequence of signal indications.
What is a phase?
A part of the signal cycle allocated to a traffic movement or to a combination of traffic movements.
What is an interval?
Any division of the signal cycle during which the signal indications do not change.
What is the purpose of the amber light?
It provides clearance time so that vehicles which entered the intersection area by the end of the green period can clear off before the conflicting stream is released.
What are the main requirements of a traffic signal?
To draw attention, provide meaning, provide time to respond, and involve minimum waste of time.
What is a fixed-time signal?
A signal set to repeat a regular cycle of red, amber and green, with phase timings predetermined from traffic studies. It does not respond to changes in traffic flow and needs no vehicle detection.
What is the main drawback of a fixed-time signal?
Flow on one road may be almost nil while the cross road is heavy, yet the time given to the cross road may still be less, because the signal cannot sense actual conditions.
What is the difference between semi-actuated and fully actuated signals?
A semi-actuated signal responds to vehicles detected on some but not all approaches, typically where a minor road meets a major road. A fully actuated signal is influenced by volumes detected on all approaches, and suits intersections of two major streets with substantial variation through the day.
Why may rear-end collisions increase with signals?
Because signals require vehicles to stop where they previously kept moving, and sudden stopping raises the risk of being struck from behind — even though total accidents fall.
What are the four types of signal coordination?
Simultaneous system, alternate system, simple progressive system and flexible progressive system.
Which coordination system is the most efficient?
The flexible progressive system, in which a computer automatically varies the cycle length, cycle division and time schedule at each signalised intersection.