"People blamed it on racing driver Jack Sears. Known as “Gentleman Jack”, the first British Saloon Car Champion sparked outrage in 1964 when he chose to test his Le Mans 24 Hours-bound AC Cobra Coupe on a public road."
"Teammate to Jim Clark, Sears started his run at the Blue Boar services (now the Watford Gap site) on the M1, just after 4am. Properly dressed in a blazer and tie, the Norfolk farmer would have encountered little traffic as the 4.7-litre AC roared ever onwards towards sunrise.
Britain didn’t possess a circuit that could replicate the famous Mulsanne Straight at the circuit hosting the Le Mans 24 Hours, so a newly-built motorway in Hertfordshire seemed the obvious answer. His Cobra wasn’t equipped with a speedometer and only when Sears returned to the service station did his team reach for their slide rules. The car had clocked 185mph."
"Unfortunately, news of the road test leaked, making front page news in the Daily Mirror. The Ministry of Transport ordered an inquiry and 18 months later, on December 22 1965, a 70mph “temporary” limit was imposed on all unregulated roads for the first time.
The new rule effectively created a national speed limit, which was extended four months later by the newly-installed Minister of Transport, Barbara Castle (who couldn’t drive). Two years later, 70mph became law, while the national speed limit on single carriageways was lowered to 60mph in 1974.
Castle had called for a profound change in public attitude to road safety, but what were the factors at play and was Sears really responsible?"
The 1965 experience:-
https://archive.ph/uYhHb
