6.12.1 Seagull T-intersections
The purpose of a ‘seagull’ treatment of a signalised T-intersection is to avoid stopping through vehicles on the major road, approaching from the left of the T-intersection stem as shown in Figure 6.21(a). This through movement is not signal controlled and operates continuously as shown in Figure 6.21(b). However, this movement could be signalised in order to make provision for pedestrians crossing. When there is pedestrian demand, the through movement would be stopped when the side road movement operates, e.g. in Phase C in Figure 6.21(b).
Generally, traffic signals should be installed on seagull intersections only where right turn vehicles from the stem of the T-intersection do not have to merge with through traffic on the departure and weave across through traffic to turn left just beyond the signals. Any merging by these right turn vehicles can result in rear-end collisions.
If traffic has to merge on the departure, the safest option is for through traffic in the left-most lane to merge to its right. This means providing right turn vehicles from the stem of the T‑intersection with their own lane or lanes on the departure as shown in Figure 6.21(a).
A capacity and performance evaluation should be carried out to determine if the seagull operation is more efficient than other intersection design options. This should account for lane under‑utilisation on the major approach road from the left of the T-intersection that is likely to be caused by this treatment. For key traffic management selection considerations at seagull T‑intersections, refer to Part 6 of the Guide to Traffic Management (Austroads 2019c).