Traffic from C-B blocks C-A traffic: This is the most important parameter when considering right turners. There is more info on this in the User Guide, but the basic principle is that: when right turners are specified as blocking straight ahead traffic, the capacities of each traffic stream are modified, and thus the output from the model will differ significantly from the non-blocking case. Whether the right turning traffic is blocking or not depends on the particular nature of the junction rather than the geometry as entered into the program. This is why the program asks you to input this value rather than computing it from the geometries and flow information.
The reason that 2.2m is entered for the right-turn lane when there is no actual right-turn lane is that PICADY always uses an ‘effective’ right-turn lane, whether one physically exists or not. In the case of a single physical lane, the 2.2m simply represents the average road width taken up by traffic turning right. A better definition is “lane width for non-priority streams”; it is important to remember that, in general, PICADY operates in terms of traffic streams rather than lanes. This means that as far as PICADY is concerned, it is irrelevant whether the various traffic streams are segregated into different lanes or not. (Also remember that PICADY assumes that the priority streams are uninterrupted (or rather, are either ‘blocked’ or ‘non-blocked’), does not calculate their capacities and so does not need to know the width of each individual lane.)
Where islands are present, the measurement of the carriageway width does exclude the right turn lane, because in these cases the right-turn lane is a separate, distinct feature of the junction that exclusively carries right-turning traffic and so is not incorporated into the general model of the junction. They also affect the minor arm capacities, as mentioned below.
All stream capacities depend on the total carriageway width, but only the traffic turning right from the MINOR road (B-A) is affected by the central reserve width (if any). When you specify that “Traffic from C-B blocks C-A traffic”, these equations are modified to take account of the extra delay experienced. This is all based on evidence used in the original research to formulate the models.
The upshot of all this is that to PICADY, a T-junction with a right-turning lane of width 2.2m is identical to a T-junction without a right-turning lane, assuming that the total carriageway width is the same.