How deep would you have to bury a 5000 volt acwu cable under a roadway. Does anyone know what section or rule pertains to this in the cec?

Section 36 deals with HV installations but I can only see where it mentions in a building , not underground distribution at all. But it says it has to be marked every 3 meters

Section 12
Rule 12-012 Appendix B and table 53.This says 1000mm. If you have any doubts check with your local inspector

I don't know beans about the Canadian code... and I consider 480v to be "high voltage."

However, I suspect my local utility would require such a line to be buried at least 5 ft. deep, especially if there was no other protection (such as pipe).

From Table 53 conductors over 750 volts must be buried a minimum of 1 meter or 39 1/2 inches for the metrically challenged.
High voltage is over 750 volts so the 480 volts is "low" voltage.