What ended the highway man period?

The railways, really.

Highwaymen targeted the coaching trade, robbing the travellers of their valuables (the poor majority walked, of course, so anybody in a coach would be of some substance and status), and making off Into the Woods. They relied on the fact that they could outrun the coach horses who would hopefully be tired, and that is why at many hotspots there would be a coaching inn a mile or so beforehand, where the coachmen would change horses, just in case they were raided.

Now a railway locomotive is as fast as any horse, but doesn't get tired. It can keep going as long as the fireman can keep shovelling. Added to this, the nature of the railway track made it almost impossible to launch a raid on the things. What with all of this, the railways caught the public imagination and within a few years killed the coaching trade stone dead. As an example, Maidenhead was an important coaching town on the main Bath road out of London to the west, with 70 coaches a day passing through. Within two years of the railway driving through towards Bristol, that number had dwindled to three. As the coaching trade died, so did the opportunities for highwaymen. At this sort of time (1830's), an organised police force was beginning to evolve with the aim of catching highwaymen and their like, and the end was nigh...