Is it true that motorways originally had no speed limit?
Yes, I'm told that coaches used to travel at 90 mph and it wasn't uncommon for cars to travel at 100 mph, that all came to an end when a temporary speed limit was introduced in winter owing to bad fog causing many crashes (as if anyone would do such speeds in heavy fog anyway). That "temporary" speed limit is still with us today.
I live one mile from the M1 and when it opened I used it every day for over ten years to commute to work for a major car and truck maker. Later, I commuted into central London on it.
In the early years Midland Red coaches were often seen travelling at 80+ mph. I drove company cars, including police demonstrators and executive models at up to 100 mph on many occasions and an occasional truck at 70+. However, there was much less traffic on the M1 in the 60s and, generally, most vehicles travelled at 55-60 mpg as cars were much less powerful than later models.
.