Making a legendary architect for your roads, bridges, and wells takes a while. The hardest part is actually designating the things to be built.
Bridges seem to provide the most experience, but you then end up with your masons or carpenters going out to finish them afters the architect designs them. Supports provide less experience, but are finished by the architect. I have no idea how much experience roads provide.
For bridges and roads, make them 1x1. I usually designate a 10x10 square full of bridges and then deconstruct them when finished. It takes five to seven rounds of this to reach base legendary, if I remember right. Though you should continue on past that.
The easiest way would be to lock the architect in a large room with lots of food, water, and 100 stones. And a bed and decorations to keep him happy. This prevents the bridges from being finished if you don't make the architect a mason. This should also prevent problems with making a macro of the designation process. This way you don't end up with your architect going all the way across the map to get a stone. When the bridges are done being designed cancel them. Then activate the macro to repeat the process. Though you may get the occasional "object blocking site" message on the later repeats, since the stone is now randomly dropped int he construction site.
Note that a masterwork-designed well made out of as many artifacts as possible produces a LOT of value. Also, make sure the mason that builds it is also high legendary. The construction quality of the mason also works on roads, bridges, and wells.