Speed limits cause endless debate – but it’s rare you see spectacular evidence as to why, sometimes, they can be a really bad idea.

That’s what happened to me late on Tuesday, travelling west on the M42 in one of the numerous roadworks zone that the imminent end of the financial year has brought about. Like an increasing number of zones in motorway work areas, it was guarded by average speed cameras. And, as always, traffic slowed to a constant 52 mph crawl.

Ahead of me a lorry rumbled along in the slow lane – or Lane 1 as we’re meant to refer to it. Adjacent to it, in the middle lane, sat a Fiat Punto that – for reasons best known to itself – was exactly matching the lorry’s speed and sitting in its blind spot.

As we approached a junction the inevitable happened. Another car appeared at the end of the truncated slip road, intent on merging with the flow. The result was no less alarming for its predictability. The HGV driver failed to realise he had a Fiat in his blindspot, the Fiat driver failed to notice the lorry’s indicators, the wagon pulled out and – after losing his door mirror, which almost took a chunk out of my Mondeo – the Punto pilot swerved right into the (mercifully vacant) fast lane.