er, actually, there are 3 routes. Go to
http://www.viamichelin.com/viamichelin/ ... oller/Maps and type in 'St Jean Pied de Port' and France.
The main road route is the D933 via Valcarlos (the Carlos concerned is Charlemagne, who is supposed to have used this route - and been ambushed on it - on the return from his Hispanic campaign). As Sil says, this is the bad weather route, as it's much lower.
The 'Camino route', waymarked with yellow arrows, is the D428 via the Col Bentarte. This is the route most people use, and as you can see is also on a road for most of the way up. It was used by Napoleon in the Peninsular Wars - coming from Bayonne, and rather better documented than the Charlemagne one.
You can see that the GR65, red/white stripes, uses a different exit from St Jean and joins the Camino route further along, though I don't think many people use it.
The 'original route' was the Roman road; this came up the valley to St Jean le Vieux (Imus Pyrenaeus) and continued in a straight line to St Michel (which is where the Codex Calixtinus route starts) and on to the Bentarte route and the Port de Cize (Summus Pyrenaeus).