You're not wrong. About 100 readers have completely misinterpreted what I wrote, most of them quoting ‘rules and regulations’ (not laws), and giving me the impression that if vehicles emerging from sliproads do not give way to them they feel obliged to assert their right of way and plough straight into the 'offending' vehicles. They either don't understand Common Sense or don't have any. In answer to your own question, the Highway Code advice that sliproad traffic “should” give way rather rather than "must" give way is because there are many occasions when this is unrealistic. Most UK sliproads are very badly designed and drivers already on the motorway have a far better view of vehicles on the sliproad than drivers on the sliproad have of vehicles on the carriageway. From a car on the motorway, a drivers view of vehicles on the sliproad is mostly side and front. From a car on the sliproad the driver's view of vehicles on the motorway is mostly side and behind, requiring craning of the neck to ensure something isn't in the blindspot.