Yes, you should be able to stop in the distance you can see and take all precautions to make sure your path is clear before proceeding. It does depend on road layout, road markings and street signs. It may well be that they were in a lane that they shouldn't have been, so it's worth checking. The case law around this scenario generally falls around similar incidents with motorcycles, i.e. Pell v Moseley (2003). The Claimant was riding a motorcycle along a single carriageway with one lane of traffic in each direction (60mph speed limit). The Claimant was travelling behind the Defendant, whom he intended to overtake. The Defendant intended to turn right into a field to attend a motorcross event. As the Defendant attempted to turn right, a collision occurred between the Claimant and the Defendant. At the first trial of the case, the judge found fully in favour of the Claimant. The Defendant appealed and, on appeal, both parties were found equally to blame and liability was awarded on a 50/50 basis. The Court found the Defendant negligent as they had failed to indicate and failed to notice the presence of the Claimant’s vehicle before turning right. The Claimant was negligent in failing to notice that the Defendant’s vehicle was slowing down and, given the Claimant was aware of the event on the right, he should not have attempted overtake when he did.