Is it possible? Oh, sure. I mean, heck, if we want to go the magic route, simply using a lot of Wall of Stone spells could make one HELL of a road. But, most people aren't real thrilled about that level of magic in the setting.
Is it plausible? That you have a very expensive, highly traveled route stretching from Toronto to Miami (a tad farther than Boston to New York mind you) through monster infested territory, considered some of the most dangerous lands in Faerun, when all the major points on the road are all major sea ports. I don't think so.
So you really believe the following:
tries to patrol the roads to try to make them safer. No ranger patrol, no marshals employed by the local nobility, no interested religious groups or secular factions who see it as their duty to protect travelers. Nobody.
has the money and inclination to use a boat, even though the oceans are equally dangerous and monster-infested, plus you can run out of fresh water and possibly drown or have all of your goods fall to the bottom of the sea.
That this road is always in perfect condition and made out of the most expensive of materials, and can't possibly
consist of local roads that have been connected over the centuries and given the name "The Trade Road" and is in various states of repair or disrepair depending on the location.
By the way, are you aware that this road doesn't stop in Baldur's Gate? It continues down to Calimport. If Calimport wanted to take a boat to Waterdeep, it would have to go through incredibly treacherous waters filled with "shoals, currents, small exposed or submerged islets, and other hazards [...] most of the islands were surrounded by perilous reefs." (So sayeth the FR wiki) The wiki goes on to say that Asavir's channel is a well-traveled sea lane but treacherous because of the many pirates.
Hmm, it's almost like this road was at least partly made so that people could travel between Calimport and Waterdeep without having to go only
through incredibly dangerous waters and risk losing their entire cargo. Baldur's Gate is just a stop along the path.
Actually, by your logic, nobody should be using either road or
boat to travel, because both Waterdeep and Calimport are famed for their Spelljamming ports
. They have the two largest ports in Faerun! Everyone should be traveling and trading by spaceship!