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 the name given in the United Kingdom to a set of rules that were first published in 1931 and designed to promote road safety. These rules became the official road user guide for Great Britain, having been updated many times.