There's a low bridge at the bottom of Kilbowie Road so the 1/1A/1B/1E are single deck because of that and the 1C/1D can be extremely busy even off peak too so it's understandable as to why they're mostly double decker. The 1C is the closest bus to me and it can be just as busy off peak as any other service I've used in Glasgow
1 is one of the few routes i think thats built up passenger numbers , other being 77.