I think first of all, a better understanding of what you mean by "drive on sight" - do you mean operating on shared rights of way (i.e. pure street running or center median with frequent grade crossings), or do you mean on private right of way either with no block signaling at all, or signals that are not enforced by some kind of automatic speed/train control system?
Also, keep in mind that "North America" is quite broad too - regulations vary wildly between the United States versus Canada versus Mexico, and in the case of the US, even between states, agencies, etc.
OK, so let's look at the first assumption - street running. That one's easy, generally a light rail line would adhere to the civil speed limit of the road it's running on, although in practice they'd probably run a little slower and with caution because LRVs are comparatively big and people + automobiles = idiots. The traffic lights perform the role of a "signal system". With a reserved side-of-road or center median ROW, speeds might be higher if the ROW is adequately fenced and grade crossings protected by lights/gates/etc.
Otherwise with private ROW but no signal system (rare but not unheard of, looking at you MBTA Mattapan Line) or with a signal system but no ATC enforcement (a lot of those exist), I don't think there is any maximum speed standard or rule at least in the US. Speeds would be based on infrastructure and operating conditions, and in the case of no signals at all there would probably be something in the rule book that says "run at a speed that will always allow you to stop before hitting anything in front of you". I know APTA and FTA have some guidelines and recommended practices but not sure of any hard, unviolable speed limit rules that apply to all properties. Then again I'm not in the light rail biz, so don't take that as gospel.
I am aware on railroads (a much more standardized environment) there are hard, federally-established limits on how fast you can go in "dark territory" versus with basic block signals versus full ATC.
