A professional driver is someone who is paid for driving and earning a living from it, so that would include taxi drivers and lorry drivers.
This is the only sensible way to define the term, IMHO.
I can well believe that one in four drivers would be willing to use a crossing if they believed that there were no more trains due. Most people are vaguely aware that freight trains don't appear in the public timetable, but don't have a clue about all the other things that can lead to an unadvertised train running. Chuck in a bit of complacency due to familiarity, and time pressure, and risk taking is more or less inevitable. Half barriers and unbarriered crossings don't help, but even fully barriered crossings see their fair share of people trying to ignore them.
I wonder what the equivalent statistic would be for people trying to pass 'wig-wag' signals in other environments - fire stations are the most familiar one, of course, but there are others. Significantly better, I should think; in those cases, the reason for the stop signal is generally pretty obvious.
There was another study done a little while ago that found that half of HGV drivers didn't know the height of their vehicle, whilst half of HGVs were over-height; the implication being that one in four HGVs is a bridge strike waiting to happen. Really, professional drivers ought to be held to a much higher standard than private motorists. I don't get the sense that they are, though - a professional driver is reasonably likely to get away with all sorts of things that would see someone in the rail, maritime or air transport industries sacked on the spot.