Traffic Modelling

Traffic modelling is the assessment of the capacity of existing or proposed junctions to determine the impacts of developments. It is undertaken using sophisticated software and provides an accurate prediction of the impact of a change in traffic patterns/volumes.

Different packages are used depending on the types of junction to be modelled.

There are four main types of junction 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, or crossroads with two side roads - these can be opposite each other or offset/staggered from each other. Priority controlled 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.  Priority controlled junctions are modelled using Junctions 9 / PICADY.


• 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. Signal controlled junctions are usually modelled using LINSIG or TRANSYT.


• 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. Uncontrolled roundabouts are modelled using Junctions 9 / ARCADY whilst LINSIG or TRANSYT are used to assess signal controlled roundabouts. When converting an existing priority controlled roundabout to signal control it is often not necessary to put traffic signals on all arms of the roundabout.


• 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 junctions at grade separated junctions could be priority or signal controlled and could be regular junctions or roundabouts. The junctions can be modelled using the appropriate packages as outlined above.

Micro simulation models such as VISSIM and PARAMICS consider a much larger area (for example an entire City or Local Highway Authority area). These programs include extremely enhanced graphics and are useful for graphical demonstration purposes for large developments.

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.

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 of a development and a future year – usually 5 or 10 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.