‘Standard’ two-lane approach
Firstly, consider the situation where there are two lanes on an approach without ‘bays’ or flares, with an opposed right-turn movement. The main consideration is whether the offside lane will contain just right turners, or a mix of traffic. If the lane is marked for right-turners only, it would normally be expected to carry just right-turning traffic. A separate link for this movement would be required in this case. If, however, straight-ahead traffic is permitted to use the lane, then it is a question of how much right-turn traffic there is: light demand will allow straight-ahead traffic easier access to the lane, whereas heavy right-turn demand will discourage straight ahead traffic from using it. You have to decide, either by on-street observation or by some common-sense based calculation, what the mix of traffic on that lane will be. Where there is a high proportion of right turners in the offside lane, some straight-ahead traffic might be able to get to the front of the queue ahead of right-turners who might otherwise block them. The number able to do this (on average) depends on the proportion of straight-aheads to right-turners, and also on driver behaviour. But it might be enough to give a small increase in capacity which might be worth modelling with an increase in saturation flow for example. On the other hand, even if straight-ahead traffic is unimpeded by the right turners, the saturation flow will still be reduced as the right-turners divert and leave gaps in the traffic flow.
Having decided the mix of traffic in the offside lane, you need to specify the give-way parameters for the opposed movement. The way this is done was covered in the first article in this series – in this case we can (usually) effectively ignore the presence of the nearside lane.
Considering the opposition
Where there are two lanes on the approach with the opposed right-turn movement, there is a high chance that there will be two lanes opposing the right turners and these lanes might form two separate links. However, you can only specify one link as the opposing link in this situation. This might not be satisfactory; but there is a way this can be overcome. If you feed the traffic from the two (or more) opposing links into a single ‘dummy’ link, the dummy link can be specified as the opposing link (see figure 1). In some cases you may need to keep the flows into the dummy link separate in order to model them separately on the next links. This can be achieved by making the dummy link a shared link in which case only the main link needs to specified in the give-way parameters. This automatically means the opposing flow is made up of the combined main and shared link(s). To ensure the dummy link has no undesirable consequences during optimisation, or on the performance index, it may be advisable to set the stops and delay to zero. The signal timings for the dummy link have to be the same as the two feeding links. This is to ensure that TRANSYT switches from using the give-way parameters to the signal control parameters at the appropriate time (usually when either an end-lag or an unopposed stage has be specified). Set this ‘stub’ link to 10 metres in length with a high (ie non-limiting) saturation flow. Ensure you look at the cyclic flow profile graphs to confirm that the model behaviour is satisfactory and understood.
When a right-turn bay is present
Sometimes, a multi-lane approach may have a right-turn bay. If so, you need to know whether the bay is large enough to store all of the right-turn demand or not. The previous article provided a means by which the queue length can be estimated.
Once you have decided whether the bay can cope with the demand or not, the link structure can be set. If the bay can cope with the right-turn demand, the approach could be specified as two, or maybe three links. Whichever way it is modelled, the capacity of the offside lane will be affected by the proportion of right-turners in the stream. The greater the number of right turners, the fewer straight-ahead vehicles in the lane. If the right turn is not especially critical, it would be possible to use three links, with the saturation flow of the ‘middle’ lane reduced in proportion to the number of right turners. The fact that right turners can then queue in their own link in TRANSYT, whereas in reality they might not all be able to reach the stopline, may not matter too much. If, however, the proportion of right-turning traffic is high (but still not enough to fill the bay) it might be better to model the situation as shown in article 2 with one link feeding two further links representing the bay and the adjacent portion of the main lane.
If the bay does fill up, it will effectively make the offside lane a right-turn only lane (see Figure 2). The main difference between this situation and the similar situation above (ie without the bay) is that there will be some space ahead of the point where the right turners divert into their bay. This extra space can be fully used by the straight ahead traffic (whether they actually use the space is another matter).
In many cases it may prove difficult to find an ideal modelling solution to your problem. However, with the application of common sense and experience it should be possible to find a solution that meets your particular requirements.