Complex sentence: The Highway Code is a set of rules designed to promote road safety that in 1931 were codified under the title "The Highway Code" which has now become the official road user guide for Great Britain, having been updated many times.
Split into simpler sentences: The Highway Code is a British booklet of rules designed to promote road safety. The booklet was first published in 1931, and has become the official road user guide for Great Britain, having been updated many times.