ATM involves converting the hard shoulder into a normal lane during periods of high traffic flow to expand the capacity of the road and may reduce the need to widen motorways. Similar schemes have already been implemented in Europe.
When the speed limit has been lowered to or below the hard shoulder can be opened as an additional lane. To facilitate this and still maintain safety a series of refuge areas have been created around every along that stretch of the road. These take the form of lay bys to the side of the hard shoulder and contain the SOS phones within them. In the event of a vehicle breaking down on the hard shoulder, operators can close it or they can close a lane to allow emergency services access to an accident. The hard shoulder is never opened on the sections under a junction between the off and on slip roads. Close to junctions use of the hard shoulder as a lane is restricted to traffic exiting or entering at that junction.
The system makes use of Automatic Number Plate Recognition cameras which to monitor traffic flows and tailor the system. Digital enforcement cameras are also mounted on the gantries and are operated by the West Midlands Police to enforce the mandatory variable speed limits.
The M42 scheme was initially run as an experiment and a Highways Agency report into the first six months of the scheme scheme showed a reduction in journey times of up to 25% The journey time statistics can be broken down to show that northbound journey times were reduced by 26%, equating to an average reduction of 4 minutes as compared to the period when the variable speed limits were on but the hard shoulder was not being used and 9% southbound (equating to 1 minute) during the afternoon rush hour. The report also indicated a fall in the number of accidents from over 5 a month to 1.5 per month on average. The Agency did state that normally accident statistics should be compared over a 3 year period, so the initial results should be treated with caution. They also stated that no accidents had been caused by hard shoulder use as a normal lane. The report also stated that there had been a 10% fall in pollution and 4% fall in fuel consumption. The report also indicated a compliance rate of 98% to the indicated speed limits when using the hard shoulder. For comparison before the introduction of mandatory speed limits at road works, the compliance rate was 10% as opposed to 89% afterwards, showing a similar effect.
The Highways Agency surveyed drivers, stating that 84% felt confident using the hard shoulder, 68% felt better informed about traffic conditions and that around 66% wanted the scheme expanding to other roads.
The Campaign for Better Transport argued that it whilst it would reduce the need for widening schemes, it did nothing to reduce traffic and CO2 emissions. Friends of the Earth criticised the scheme as "widening on the cheap" and also pointed to a possible increase in vehicle emissions. The Highways Agency argue that ATM reduces the environmental impact in regards to widening as it is carried out within the existing boundaries of the motorway as well as a possible improvement in local air quality due to smoother traffic flow.
The RAC cited a study in the Netherlands that showed drivers using the hard shoulder when they were not permitted, increasing the risk of a collision for vehicles stopped. The Royal Society for the Prevention of Accidents also expressed concern that emergency services would take longer to reach an incident. The Highways Agency rejected this concern based on the 5,000 miles of dual carriageway which doesn't have a hard shoulder. Disability groups were concerned that some drivers would not be able to access the emergency phones or even exit their vehicles, leaving them at increased risk.
Ruth Kelly, Secretary of State for Transport stated in an interview that this would not necessarily replace motorway widening in all circumstances, but would be another option and that the government were not using it simply a cheaper method of widening roads.