What is a smart motorway? Speed cameras, safety and hard shoulder use explained

Smart motorways use variable speed limits to control the flow of traffic and, in many cases, instruct drivers to use the hard shoulder as a live running lane. Breakdowns in live lanes have had tragic consequences, though, and the safety of smart motorways has now been called into question.
This criticism is mainly aimed at all-lane running (ALR) smart motorways, where every single lane - including the hard shoulder - is used by live traffic. When these were rolled out, they were not eq