Smart motorways have led to safety concerns, but highways bosses insist they are an effective way of boosting capacity. Here the PA news agency answers 12 key questions about the roads:
What is a smart motorway?
Smart motorways involve various methods to manage the flow of traffic, including variable speed limits and using the hard shoulder as a live running lane.
How many are there?
Motorways with sections where the hard shoulder has been removed include the M1, M4, M5, M6, M25 and M62.
The RAC says the smart motorway network will cover around 500 miles this year, with an additional 300 miles planned by 2025.
Projects under construction include the M4 between Junctions 3 and 12, and the M1 between Junctions 13 and 16.
What are the benefits?
They are designed to increase capacity without the more disruptive and costly process of widening carriageways.
Are smart motorways safe?
Concerns have been raised about incidents where vehicles stopped in traffic are hit from behind.
But Highways England insists they are 'at least as safe as, or safer than, the conventional motorways they replaced'.
What does the data show?
An 'evidence stocktake' published by Transport Secretary Grant Shapps in March stated that the risk of a collision between moving vehicles is lower on smart motorways than conventional