The culvert for a farm entrance should be at least 40 feet long.  I just put in a 30 because that was all the state would write the permit for.  You need to make your entrance tulip bulb shaped

 You have the limiting width at the road entrance.  Make your drive wide enough to swing and get back down to the gate width.  If you have to hit a 15 foot road entrance then make the drive so the tractor can swing far enough for the trailer to make it.