I have two 18' garage doors on my garage, on the 42' side. I would NOT recommend 18' doors.
First - you have to have more reinforced header. Second they are heavy. Third with wind loads they have to be reinforced at least here in Florida.
IMHO - a far better solution is the two 10' doors suggested - or better still two 9' doors with a couple feet between them. Leaving a couple feet between them will give you more room to open car doors without hitting other cars - and that extra two feet in the center is a good place to put electrical and compressed air outlets.
FWIW,
Carl B.
Good points to bring up as well. I've got three 9'w x 8'h double faced insulated 2" doors and they are heavy.. an 18' would surely be a beast..
I initially had two 16' doors on the plans to be a four car, but since the garage is a full finished 2 story structure and I wanted free span with no poles, I had to either use TJI's (which would have decreased my ceiling height) or add 2 Prelam beams parallel with the bays to support the floor above, so the jackstuds holding the Prelams are in the wall space between the three doors.
I'm happy in the long run, since the garage is heated and air conditioned year round, I loose way less air when pulling a car in or out only having to open the correct door and not the whole side of the building like a 16'-18' would essentially do.