- The common date is the massive introduction of the automobile, in the early 1900s (interpreting these data) or 1908 (Ford T model production start). After the US civil war, a lot of train robberies happened, but the trains (as later the planes against high-jacking) were rapidly secured.
The car itself rapidly became more value for potential robbers than most objects private passengers could carry, 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 tattooed 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 purchases from the police) rapidly made it hard for highwaymen to act successfully.
Nevertheless, 'Highway robbery' concentrated on high-value objects (pharmaceuticals for example) and cash transport like here or like the Denver Mint robbery of 1922. However they now prefer to act at truck stops, as with car-jacking taking place on signal lamps.
A good indicator of "highway robberies" could be the percentage of robberies carried out outside of towns, if we consider the approximation that the crime rates have sunk constantly in the modern era. The actual rate according to the US Census Bureau was 3% in 2009, and 4% for motor vehicle theft. Data prior to 1960 cannot be found so easily.
Another possible answer would be the change in the perception of highway robbery : Maybe highway robbery rates just didn't develop during the 1905-1915 period which saw a huge increase in violent crime (often linked to the migration and urbanization wave in this period, even if migrants crime rates are likely to have sunk during this time, cf Immigration, Crime, and Incarceration in Early Twentieth-Century America by CAROLYN MOEHLING and ANNE MORRISON PIEHL, a very interesting paper which can be found using a search engine). This would have made less spectacular, and as traveling became more common, the fear of highway robbery faded.
