Wedge

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

A car coming from the right, such as around a roundabout, might create a wedge by leaving into the road you're waiting to come out from.  (That could helpfully hold up traffic waiting to enter the roundabout from the  approach road on your right, thereby giving you a chance to  go).

Check there's no-one hidden behind the car that's leaving, then use the space it has levered open to launch yourself out.  Be sure any car following it doesn't catch you up.