Highways Agency

Controlled Motorway

On the 29 November 2005 a controlled motorway scenario was introduced as the second phase of ATM operation. This meant that automatic mandatory signals were implemented on the M42 section junctions 3A -7.

The Signal Controls

There are two reasons for automatic signal and message sign settings:

Signals set for Congestion

These are set in response to the number of vehicles per minute passing over the loop detectors - the traffic demand. As demand increases so does the risk of flow breakdown and accidents. At carefully calculated thresholds, the signals are set to reduce the speed of traffic, smooth the flow, reduce the potential for flow breakdown and create a safer environment for the current traffic conditions. Initially, 60mph signals are displayed on the gantries and then as the traffic demand increases further, 50mph signals are displayed.

Drivers may not realise why these signals are set, and this is understandable as there may be no apparent reason for the settings. However, the system is detecting high traffic demand and is using the signals to prevent the congestion from deteriorating into flow breakdown. This makes the journey smoother and safer for all.

Signals set for Incidents

These are set when the system detects very slow moving or stationary traffic over a loop detector. The signals provide warnings to protect queuing traffic and produce a safer driving environment. When a queue is detected, the system quickly sets signals to 40mph limits in the immediate area and 60mph limits leading into this, to give advance warning to drivers alerting them to reduce their speed. If the signal gantries are quite close together, the advance signals will be set to 50mph limits. Message signs are also set with appropriate text to support this situation.

Incident settings are designed to protect stationary/slow moving vehicles and the back of queues that can result from these. The incident detection system also works alongside the congestion system to control the speed of traffic in congested areas where flow breakdown has already happened.

Message Signs

At regular intervals on the motorway there are message signs giving text information to give more details to the driver about the situation ahead and these reflect the signal activity. Examples of the automatic messages are "CONGESTION STAY IN LANE", "QUEUE AHEAD", or "QUEUE AFTER NEXT JUNCTION". Other messages such as "Obstruction" can be set manually by the Police.

Timing - why are the signals on so long?

The signal system is intelligent and prevents the signals changing the limits displayed or switching the signals on and off too quickly. This gives drivers time to respond and ensures the signals are not confusing. Once the signals are on, timing delays are introduced to stop signals switching off prematurely. Studies have shown that after heavy congestion has occurred, it is vital to control the recovery of traffic speeds and let the traffic flow recover safely. This minimises the risk of further flow breakdown or traffic incidents re-occurring.

The occurrence of shockwaves in Controlled Motorway Scenarios

Studies have shown that a shockwave occurs when the density of the traffic reaches a critical level. Unstable traffic speeds combined with sudden braking creates a shockwave which travels back through the traffic at about 12mph.

Drivers experiencing a shockwave find that they suddenly have to slow down, then a few moments later they can speed up again. The causes of this stop-start driving are varied, some are due to incidents, some are due to the physical layout of the road, such as traffic merging at a junction, and others appear to have no cause.

Frequently Asked Questions

Q: Why do I see 60mph limits set when there are low flows of traffic and the traffic appears to be moving freely?

A: This happens after a period of congestion on the motorway and the traffic conditions are recovering. A controlled recovery (60mph limits) is more likely to be sustained and improves safety. If this was not implemented, there would be a high chance of flow breakdown recurring.

Q: Why are 40mph limits often followed by sections of heavy congestion?

A: The incident controls are in operation and the system is protecting the backs of the queues as the vehicles move from shock wave to shock wave.

Q: Why do I see what appear to be inappropriate signals and messages on the gantries?

A: The system is responding to a real event that is happening at that moment, further ahead of the driver. It is possible that by the time the driver arrives at the location of the event, there is no apparent cause, and the traffic conditions have resolved themselves.