Firstly, note that an opposed right-turn link has to have both signal control data and give-way data. In the situation where right-turners do not benefit from an unopposed stage, they will turn in gaps in the opposing flow, and those waiting in front of the stopline at the end of the green will clear during the intergreen. In such cases the end lag (found in the signal control data) needs to be used to model the potential of vehicles clearing during the interstage. The end-lag time is specified in seconds and should be set as long as necessary to clear the number of vehicles able to store in front of the stopline (or, to put another way, the number that would store in front of the stopline if the approach was fully saturated). This would normally be (3600/saturation flow) x (number of vehicles stored) where the saturation flow is that specified in the signal control data (not the give-way max flow because the right-turners are unopposed during the end-lag period). Note that the end lag is a modelling feature that extends the TRANSYT effective green. It does not affect the actual green used on-street.
We often get asked what give-way parameters should be used in such a case. We normally suggest starting with a coefficient of 50 and a max flow of 900-1000. These are not to be taken as any standard figure. You should feel free to alter them in the light of experience or better knowledge if necessary. However, a right-turn movement that does not have the luxury of an unopposed period would not normally survive unless it was non-critical (i.e. very low flow) and so the give-way parameters themselves should never be critical. If they are, it probably would not be wise to persevere without an unopposed stage.
Moving on to a specific situation, consider the case where there is a single lane on the approach to the signals, with no bays or flaring, and the right-turners share road-space with other movements. In this situation, it is possible in TRANSYT to specify other movements as giving way to ‘nothing’ (leaving just right-turners giving way to ‘something’). It models the stream as-a-whole and takes account of the blocking of other movements caused by those waiting to turn. In doing so it correctly models the combined effects of other movements discharging at the signal-controlled saturation flow rate and the right turners discharging at the give-way max-flow rate. It does not allow for the fact that, normally, one or two right-turners can move into the centre of the junction out of the way of other movements (even though you can model their discharge during the intergreen with end lag). Consequently, this modelling technique may underestimate the situation. This underestimation ought not to be a concern because, if the situation is sensitive to the data, it may well be time to consider the addition of an unopposed stage for the right turners.
Whilst on the subject of opposed right turn situations, one thing you positively cannot do in TRANSYT is to have a link specified as opposed that is itself a priority link for some other opposed link (i.e. mutually opposed). Often, TRANSYT will not tolerate this and fail to run correctly. However, sometimes TRANSYT will run: if it does run, the results certainly should not be trusted. When this mutual opposition exists, it is necessary to model the capacity of one of the links (usually the one with the least right-turning traffic) by suitable adjustment of the saturation flow. The saturation flow can be calculated by using OSCADY or RR67 formulae and it is still possible to use the end lag to model turning during the intergreen. (Note that, in such cases, the end lag time calculated by (3600/saturation flow) x (number of vehicles stored) may seem excessively long when the saturation flow is low and you may need to be prepared to defend it).
In Part 2, I will continue with this theme, covering situations where right-turn bays exist and where there are two (or more) lanes involved, which adds whole new dimensions to the problem.