I used America just for the sake of inquiry, rather than saying when did we as a whole. It could be any nation that implemented workplace regulations and practices, but I thought it would give a good basis by using the United States.
My Data is for mining not construction but there doesn't seem to be a sudden drop at any point. The changes seem to be slow and steady over the last 100 years. also my data is only for fatalities not for other injuries.
The construction of the Golden Gate Bridge wasn't the first to have construction safety requirements, but it was the first to implement them so rigorously, and some elements were created specifically for this job (such as a safety net under the construction zone). Because of these requirements, relatively few people (10) were killed during construction. The rule of thumb prior to this was "one person per million spent", and the bond was for $35 million. These safety requirements were neither quickly nor universally implemented in the construction industry, but it was a turning point.
http://www.pbs.org/wgbh/americanexperience/features/general-article/goldengate-safety/ http://goldengatebridge.org/research/CheatingDeath.php http://www.constructionequipmentguide.com/Golden-Gate-Bridge-Celebrates-75-Years-of-Putting-Safety-First/18278/