Wedge

You use a wedge to force open a gap, or keep a door open.

You can look out for a wedge to get into busy traffic, like onto a roundabout or from a minor road into a major one.

As you approach a junction, look for any car coming from the right.  If it slows, it's probably about to change direction—maybe turning into the road you're waiting to leave.

Once you know there's no other road user hidden behind it, you can use the space it has levered open, to launch yourself out.  (Be careful any car following it doesn't catch you up).

If waiting to enter a busy roundabout, watch for the NEXT PAIR of cars.  If either starts exiting, there's your chance to run !