In all the games I have played so far, I have only rarely seen a trail upgrade to a path.  It just takes too long for it to happen.  Take two cities that are 12 squares apart.  A caravan should travel 4 squares per turn, which works out to 3 turns to the other city and then 3 more back for a total trip time of 6 turns.  It takes 20 trips to upgrade a trail.  That means from the time you establish your first caravan, which is usually at least 60 turns in, if not more, it takes 120 turns to upgrade a short road from a trail to a path.  So you are almost at 200 turns before there is any road upgrades.  This example is a short caravan considering the minimum distance between cities is 8.  Double the distance and you double the time to 240 turns to the first upgrade at around turn 300.   Most games are over by then.

I would rather see the trail grant 1.5x movement and upgrade faster to 2x, 2.5x, 3x, etc.  Either that or remove road upgrades entirely or make them tech based.  I like the idea of the roads upgrading by use though.  I just wish it made more sense when you have two caravans traveling the same section of road, why doesn't that section upgrade faster.  That part doesn't make sense.

Do upgraded roads increase the trade value of the caravan?  IMO it should, as should distance and population of the trading partners, and foreign trades should also be worth more.

The issue of friendly unit movement through caravans needs to be addressed too.