3-Legged Roundabouts

Creating or customizing a 3-legged roundabout block is essentially very similar to creating or customizing a T-intersection block. Here too, four separate Intvol blocks are required representing the cases with "no eastbound", "no westbound", "no southbound", and "no northbound" approaches. To allow for maximum flexibility, arrow blocks are created for every movement, i.e. left-turn, through, and right-turn.

Note: While it may intuitively makes more sense to eliminate the "through" arrow from the 3-legged roundabout block "assembly", as illustrate din the example below -

- many if not most traffic models use a generic "all-movement" intersection formats, with non-existing movements shown as "zero" volumes. This is done, again, to provide maximum flexibility. For that reason, TIAH 4 uses all three (left, through, right) arrows in the 3-leg roundabout blocks. Once the block is switched from the standard 4-leg block to the desired 3-legged block, either the through or right-turn movements (depending on how the intersection and corresponding volumes were constructed in the traffic model, sometimes the left-turns as well) will have zero values and can be hidden by toggling the zero volumes to "off".

The graphic below illustrates this point. Notice that in the block on the left, the model's volume output is consistent with an intersection without the southbound leg, i.e. there is no volume value (shown as zero) for the NBT, WBR, and EBL movements.