The common date is the massive introduction of the automobile, in the early 1900s (interpreting [these data][1]) or 1908 (Ford T model production start). After the US civil war, a lot of train robberies happened, but the train (as later the planes again high-jacking) were rapidly secured.

The car itself rapidly became more value for potential robbers than most objects private passengers as well as a very efficient way to escape the bicycle- and horse-mounted police (1910 - 1911, first systematic car thefts by the Bonnot Gang in France). In the US, reselling car parts or stolen cars soon became relatively easy (horse were tatooed and it's not easy to resell horse car parts...).

The rapid evolution of the police structure and mobility (1912 saw the first mobile police squadrons in Paris, 1920 the first systematic automobile patrols in NY, from 1932 on there were massive automobile purchase from the police) rapidly made it hard for highwaymen to act successfully for an always .

Nevertheless, 'Highway robbery' concentrated on high-value objects (pharmaceuticals for example) and cash transport like [here][2] or like the Denver Mint robbery of 1922. However they prefer to act at truck stops, as with car-jacking taking place on signal lamps.


 


  [1]: http://hypertextbook.com/facts/2001/MarinaStasenko.shtml
  [2]: http://www.nytimes.com/1998/09/12/us/national-news-briefs-defendant-pleads-guilty-in-17-million-robbery.html