I suppose that primarily depends on whether you need planning approval for it. I have two bridges.. one is a right of access from my land to another's filed and is essentially a couple of RSJ's spanning the thing and then planks across those. But the guy whose land they go to has left it neglected for decades and i woudnlt trust it for more than foot traffic along a girder. He takes a quad bike across those rotten planks and tin-sheets that i wouldn't tread on.
The other good bridge crossing between my own fields is basically a huge concrete pipe section sitting in the stream and then gravel filled to level it and fenced - simple effective and safe for large tractors to cross.
If you need planning for any reason then any architect should be able to put you in touch with appropriate engineers - just wait until the bill arrives though...
pgk