"Dan Luke" wrote in message
...

Then I've committed an awful lot of runway incursions without
ever being told about it.


Well, you have if you crossed the assigned runway. If you merely crossed
other active runways that were not assigned to you then you're okay.


§ 91.129 Operations in Class D airspace.

(i) Takeoff, landing, taxi clearance. No person may, at any airport with an
operating control tower, operate an aircraft on a runway or taxiway, or take
off or land an aircraft, unless an appropriate clearance is received from
ATC. A clearance to "taxi to" the takeoff runway assigned to the aircraft is
not a clearance to cross that assigned takeoff runway, or to taxi on that
runway at any point, but is a clearance to cross other runways that
intersect the taxi route to that assigned takeoff runway. A clearance to
"taxi to" any point other than an assigned takeoff runway is clearance to
cross all runways that intersect the taxi route to that point.