Or how about too narrow roads at unremittingly high speeds, with not difficulties to break up the bunch, in the early stages of a Tour that puts excessive performance presure on ALL riders...and you have the disaster scenarious we have witnessed thus far. Poor course design does nothing to mitigate the likelhood of mass crashes, rider presure and nerves do the rest.
But it will always be a question of balance.
Sure, at one extreme, you could ride an entire stage on the same motorway - or for that matter on a F1 track - from end to end, but it would be incredibly boring (and incredibly hard for spectators to get to, if it's the motorway example).
On the other hand, the more warrying terrain, of warrying quality, you use, the more dangerous the race becomes, but also the more exiting to watch.
I don't think a (wide) 2-lane road is particularly abnormal in cycling, nor particularly dangerous.
It's just that the pressure is so much higher in the Tour, that every riders tries a little too hard.