We drove the interior route to almost Belize a couple of years ago. You can see that trip on our blog starting
http://jacksonsjourneys.blogspot.com/2011/11/october-30th-last-night-in-usa-and-on.htmlLast year we entered Mexico from a different location and followed a similar route for part of the way then turned south--did not approach Belize. The blog for that part of the trip starts part way down
this page.I must agree with others that the coast roads in MANY PLACES are terrible. From Veracruz down were much better.