Highway Code Rule 259
Joining the motorway (rule 259)
Joining the motorway. When you join the motorway you will normally approach it from a road on the left (a slip road) or from an adjoining motorway. You should
- give priority to traffic already on the motorway
- check the traffic on the motorway and match your speed to fit safely into the traffic flow in the left-hand lane
- not cross solid white lines that separate lanes or use the hard shoulder
- stay on the slip road if it continues as an extra lane on the motorway
- remain in the left-hand lane long enough to adjust to the speed of traffic before considering overtaking.