Traffic congestion on the motorway network can have serious effects to the economy, quality of life and environment. By employing new technology, safe and reliable traffic flows on the motorways can be achieved. Traffic modelling plays a key role in assessing the economic, environmental and safety benefits of implementing this new technology.
Fox Traffic Simulation has the modelling skills necessary to build state-of-the-art motorway models to assess the latest technology.
In order to smooth the traffic flows around the entry slips onto motorways a technique called ramp metering can be used. Traffic signals are used on the on-ramps to adjust the flows entering the motorway.
The Highways Agency have produced an Interim Advice Note (66/05), that provides advice upon the circumstances where ramp metering may be of benefit in improving the flow of traffic on the main line carriageway of a motorway, in the vicinity of an entry merge from a slip road or interchange link at grade separated junctions. The following explanation of the problem that ramp metering addresses is taken from the Interim Advice Note.
When traffic flow on the main carriageway is high this means that the traffic concentration is also high. The concentration is the amount of carriageway covered by a vehicle. A high concentration directly equates to smaller distances between vehicles or a low headway, in such conditions the road’s capacity is close to being reached and small changes in the nature of the traffic flow causes it to become volatile and susceptible to flow breakdown.
The introduction of traffic from the on ramp can cause vehicles to change lanes and bunch leading to higher concentration and lower headways. These shorter headways can be unsustainable at the speed of the main carriageway, for comfort and safety drivers will adjust their speed to account for the short stopping distances available. This adjustment of headway occurs over a distance of up to 2km after the on slip.
Often this adjustment of headway will cause following vehicles to brake, propagating a “wave” of braking vehicles in the traffic stream. Traffic concentration in the wave will be even higher. To compound the problem more vehicles will be entering the main carriageway boosting concentration even higher. If vehicles continue to join, ultimately the main carriageway speed will drop to a point where flow breakdown occurs. In this situation vehicles are stopping at the back of a queue and then driving off the front of the queue. This stationary traffic is typically seen between the merge area and approximately 2km downstream.
As a result of standing traffic, sometimes called a "phantom jam", the road effectively has its lowest throughput when the demand is at its highest.
Weather conditions, daylight, vehicle mix and gradients amongst other things can all affect the maximum throughput of any section of motorway.
To address this problem, ramp metering aims to maximise throughput on the main carriageway without disrupting the local road network. It does this by controlling the discharge of traffic from the slip road to reduce the interference of merging traffic on the main line flow thereby maintaining speeds at a higher level. Maintaining higher speeds will postpone the onset and duration of flow breakdown on the main carriageway. To do this it relies on the measurement of traffic conditions on the main carriageway and attempts to maintain this at a 'target occupancy' by restricting the flow from the on ramp.
By having stop lines with signal heads on the slip road, merging traffic can be held back as the target occupancy on the main carriageway is approached. The slip road behind the stop line is used as a buffer allowing traffic to be stored temporarily and then ‘metered’ out onto the main carriageway.
Ramp Metering has been used in the UK on the M1, M3, M6, M62 and M27 motorways and further use is expected in the near future.
A Table is provided in the Interim Advice Note of Criteria to be satisfied before Ramp Metering is considered for installation. However for detailed assessment of the likely benefits of Ramp Metering it is useful to use a micro-simulation model such as Aimsun Micro. Fox Traffic Simulation can develop such models.
Incidents on a congested road network can quickly become a major cause of delay. Aimsun is able to model the effects of incidents so that traffic management options can be found that minimise the disruption.
Aimsun has a built-in methodology to make it easy to generate incidents and compare different management strategies for dealing with them. Within a Scenario it is easy to use building blocks called triggers and actions to create powerful strategies. Actions include rerouting by origin, destination and vehicle type; modification of speed limits; lane closure by vehicle type; incident creation. Actions can be triggered at set times or in response to a variety of traffic conditions anywhere in the network.
Aimsun can be used to study incidents and develop incident management strategies off line, or can be used on-line as an integral part of an incident management system to provide short term forecasts in real time in response to events on the road network.
Here is a simple example of off-line incident management from an Aimsun Micro model of the motorway network in Kent. An incident on the M2 results in one lane of the eastbound carriageway being blocked causing severe delays. The congestion triggers a VMS which directs traffic off the M2 at Junction 5 around the incident via the A249, M20 and A229 before rejoining at Junction 3.
Aimsun lets the user specify an incident that will block a lane on the M2 at a given time for a given duration, in this case starting at 08:30 and lasting 45 minutes.
It is possible to define triggers that turn on and off actions. In this case triggers are defined that look at the average speed of vehicles that cross a westbound detector between junctions 4 & 5 on the M2. If the speed at the detector drops below 80 km/h (50 mph) then the trigger can be used to activate a strategy. A similar trigger can be used to deactivate a strategy, in this case if the average speed at the detector rises back above 90 km/h.
In this case the trigger is used to force traffic heading east towards London off the M2 at Junction 5 and on to a predefined diversion. A VMS is used to inform drivers to take the diversion.
Aimsun also makes it very easy to analyse and compare the results of the different simulation runs to assess the incident management strategies. Here are the Time Series outputs from Aimsun showing how the overall network speed varies throughout the AM Peak according to whether there is an incident and whether the incident management strategy is applied. The results are average results from ten replications with different random number seeds.