http://www.ontario.ca/laws/statute/90h08
"highway" includes a common and public highway, street, avenue, parkway, driveway, square, place, bridge, viaduct or trestle, any part of which is intended for or used by the general public for the passage of vehicles and includes the area between the lateral property lines thereof; ("voie publique")
Rate of speed
128. (1) No person shall drive a motor vehicle at a rate of speed greater than,
(a) 50 kilometres per hour on a highway within a local municipality or within a built-up area;
(b) despite clause (a), 80 kilometres per hour on a highway, not within a built-up area, that is within a local municipality that had the status of a township on December 31, 2002 and, but for the enactment of the Municipal Act, 2001, would have had the status of a township on January 1, 2003, if the municipality is prescribed by regulation;
(c) 80 kilometres per hour on a highway designated by the Lieutenant Governor in Council as a controlled-access highway under the Public Transportation and Highway Improvement Act, whether or not the highway is within a local municipality or built-up area;
Colloquially when we think highway, we think of a grade separated controlled access freeway (ie. 401, QEW, DVP). That's not the way highway is defined in the highway traffic act.
Where 50km/hr and 80km/hr applies is also defined. If it's a rural area, then automatically the default speed limit is 80km/hr.
There are signs at city/town borders that clearly state default speed limits as well.
Toronto: https://goo.gl/maps/3xka1
Caledon: https://goo.gl/maps/ou2Lz