You'll want to always make your trade route at least 5 or 6 tiles wide rather then just 3 tiles wide. Don't forget to smooth all boulders and cut trees. Paved roads also help out, but you can cheat and just put down a custom stockpile which accepts nothing which prevents tree growth. I usually use a stockpile designation to help with layout/design, then replace it along the way with a paved road once enough stone blocks have made it to the stockpile by the construction site.
My trader entrances are usually 7-wide, with a 5-wide paved road down the middle. That way, two wagon trains can get past each other easily. I'll have a few bridges which might only be 4-5 tiles wide, but most of the route in will be 7-wide. Some areas, such as the outer forts which overlook the trade route, might be 9-11 tiles wide.
I also try to have (eventually) two or three routes into the Trade Depot along with putting the depot in a location with empty area on at least two sides. That way there's plenty of room for the wagons to position themselves and get around each other.
The simplest, mostly foolproof design for a trade road is a big long U-bend, with at least 2 bridges along the way that you can control. Make sure enemies have a shorter route into the fort then taking the U-bend, and if some of them start taking the U-bend, raise one of the bridges. Length of the U-bend should be about 15-30 tiles.