SRD, this is most prescient.
My route goes from Gilmore Place to Leven St to Valleyfield Street, so I almost never exit from Tarvit Street. Yesterday having missed the first green I was at the front of the queue for the next green. The problems of car drivers 'trying to get through' are rife.
Drivers turning right from Home Street into GP will attempt to do so for as long as possible, which can delay drivers leaving TS. Yesterday there were so many vehicles coming out of TS that my green light went back to red while I was sitting just shy of the box junction markings (and therefore beyond the stop line), and five more cars came out of TS and crossed the junction into GP. Were they all chancers? I took my chances after that and got the hell across the junction.
It made me wonder whether the duration of the green light out of GP was actually 5-10 seconds shorter than that out of TS. There is also some sort of psychological design of the junction that seems to give automatic priority to vehicles from TS, since almost no-one ever turns left from there.
And, SRD, make sure your Sustrans person positions themselves right at the box junction so that they nearly get squished by right-turning buses and then Tarvit St cars.