There is enough guaranteed storage underneath the seat that the lowest tier ticket guarantees, overhead sized luggage has become an extra fee for the last ~7 years.
The zip does nothing on the roundabout.
bumper to bumper is very-rare, and it mostly happens momentarily due to pedestrian crossings placed directly after the entrance/exit of the roundabout.
What is actually super dangerous are the non-standard 2/3 lane roundabouts where the first lane gets "ejected" at the next/second exit forcibly, while the 2nd lane can either exit or continue on the roundabout, so many lorries in the 1st lane try to runover those in the 2nd lane trying to exit.