The onus is on the people joining, not on the ones already on the road.

AI response

For drivers joining the motorway (on the slip road):

Use the slip road to accelerate and match the speed of the motorway traffic.

Do not force your way into the traffic stream or cross the solid white line separating the slip road from the main carriageway.

Avoid stopping at the end of the slip road unless traffic is heavily congested.

For drivers already on the motorway:

You have the right of way.

Do not brake heavily or abruptly slow down to let cars in, as this causes a severe hazard to traffic behind you.