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Something tells me you haven't quite thought this through :-) Take an example: there's a road I drive frequently, about 17 miles between place A down in the valley and place B at the top of a mountain, with an elevation change of about 4400 ft. So you're going to build two completely different roads, one for downhill and the other for uphill, and (using your 30 miles at 0.7% = 1100 ft, which I haven't checked) the downhill road is going to be 120 miles long.
Now suppose you happen to live halfway up the hill, or want to visit a place (e.g. the local ski area) that's partway up. You either have to waste a lot of time going one way in order to go the other, or have to build a lot of connector roads between the up & down routes.
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Large downward elevation shifts are not precluded, it is never worthwhile traveling additional distance on lesser downward gradients to get to a lower point.
When large downward elevation shifts are undertaken most advantage from the 0.7% gradient will be lost, the energy stored as height will be lost through braking rather than assisting in traveling.
These substantial downward elevation shifts are not typical. Glasgow is a hilly city, of the 20 cities I have looked at it is the hilliest. Within 20 km it does rise to 400 metres to the north and 300 metres to the south, Traveling in from both of these locations would require substantial downward elevation changes.
However this is not typical. About 1/2 of the circumference of a 20 km radius circle from the Glasgow's centre is below the 140 metre elevation required to give a 0.7% gradient. Major inbound feeder roads could use this terrain effectively to provide a 0.7% gradient from 20 km out.
Here is a Google Earth Overlay showing the gradients from locations in the Glasgow region to the city centre
If anyone would like Google Earth gradient overlays for any other cities I can produce them simply, just add a post.
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