11.3.6 Merging of High-speed Major Roads
A different type of operation applies where it is necessary to merge two high-speed roads that are of similar importance – referred to as a branch connection. In this case, the two roadways meet tangentially instead of at a slight angle as with a ramp (1:50). The angle associated with a normal ramp is intended to provide a cue for an impending lane change which is not the case in a treatment for two high-speed roads.
Figure 11.8 illustrates the principle of merging two major roadways, where the layout is based on achieving a high standard of alignment for both high-speed movements. This arrangement may be used at a system interchange.
It is very important that right-hand merges are avoided in greenfield projects (preferably re‑designed at other sites) because of the safety and operational problems they cause. This means that the merging streams should have their own lanes after the merge and any reduction in the total number of lanes beyond the merge should be achieved in a manner similar to that shown in Figure 11.8(b), using a left-hand merge on the outermost left-hand lane. If the right-hand branch carries significantly lower volumes than the left-hand roadway, then an acceptable arrangement is to bring the right-hand roadway over the left-hand roadway and create a normal left-hand merge (refer to AGTM Part 6 (Austroads 2013a).
- T = taper length based on a rate of lateral shift of 0.6 m/s.
Source: Based on VicRoads (2011).