Traffic Modelling

Four main types of junction are found on the highway network. These are:

• Priority controlled - where traffic flow on one or more side arms gives way to the main road. These can be in the form of simple “T” junctions or “T” junctions with a right turn lane on the main road and flared minor road approaches. Priority junctions operate efficiently until the main road flow is too high to provide gaps for side road traffic to pull into, thus causing excessive queues on the side roads
• Signal controlled – all arms are controlled by traffic lights that indicate which approach has the right of way at any given time. This ensures that side road traffic is given the opportunity to cross/join the main road flow. These junctions may also include dedicated signals for pedestrians which can provide substantial safety benefits to pedestrians in the overall design
• Roundabouts – these may be either conventional priority (uncontrolled) or signal controlled and work best where the flows on all approaches are relatively balanced. Adding traffic signals to existing priority controlled roundabouts can in some circumstances increase capacity and reduce queues
• Grade Separated – the turning movements are taken away from the main road flows, for example a motorway junction with a roundabout either elevated above the motorway or with the motorway elevated above the roundabout. The key element in this design is that main road flows do not have to be stopped to allow the turning movements to occur thus reducing delay to the main road and retaining high volumes of main road flow in a safer environment

The computer modelling package used for traffic modelling to assess a junction depends on the type of junction selected and the most commonly used modelling packages and their uses are as follows:

• PICADY - used for priority controlled junctions such as T-junctions and crossroads.
• ARCADY - used for priority controlled roundabouts, including mini-roundabouts.
• LINSIG - used for all types of signal controlled junctions, including signal controlled roundabouts. LINSIG is used for both standalone junctions and networks of two or more junctions.
• TRANSYT - used for signal controlled networks comprising of any type of signal controlled junction. TRANSYT cannot be used for standalone junctions.
• Micro simulation models such as VISSIM and PARAMICS consider a much larger area (for example an entire City or Local Highway Authority area) and are mainly used for graphical demonstration purposes for large developments such as retail parks.

The outputs from the traffic models give an indication of the expected level of queuing, capacity and delay at a single junction or a number of junctions linked together. Where approaches are shown to suffer excess queuing or are over capacity, the designs are amended as far as possible to bring these values to a more acceptable level by means such as addition of extra lanes or the introduction of traffic signals or improving the existing “controller” which operates the signals. These changes are then reflected in updated models.

For a usual Transport Assessment submission, traffic modelling will be undertaken for the base situation – i.e. current traffic flows on the current layout, and then for an opening year and a future year – usually 5 years after the opening year. Where necessary, these models will include any highway works, such as widening, to give an indication of the impact of the development in the future.

Where traffic signals are shown to be operative at or above capacity or have a problem with excess queues, a number of techniques can be used to reduce these problems. These can include changing the sequence in which the traffic signals operate or installing a more adaptive control method such as SCOOT or MOVA which can react better to the observed queues and demands to reduce the queues and increase capacity.