The original note, as written at the time
UNDULATING MOTORWAYS, CONTINUOUS BRIDGING PEDWAYS. The motorways should be the ones trying their best to fit within the city, not the pedways. In this urban mobility version, the motorways would from time to time descend below the city level to make way for pedestrian crossing bridges. This would mean that the pedways would never have to radically change slope.
Ongoing development
Motorways should be the ones trying to fit within the city, not the pedestrian bridges. In this version the motorway descends below the city level from time to time, to make way for pedestrian crossing bridges — so the footpaths never have to change slope radically.
That last clause is the whole policy: the walking surface stays level and continuous, and the fast surface takes the undulation.
Anyone who has pushed a pram, a trolley, a wheelchair or a bicycle up the ramp of a pedestrian bridge knows exactly what is being solved. The bridge exists because the road took the ground, and then it demands that the pedestrian climb, which is a tax on the wrong party.
Making the road dive instead of the walk rise inverts the cost. It is more expensive per crossing and it is unambiguously better for the city, because the pedestrian network becomes a level plane and the road becomes the thing that adapts.
The engineering is highway geometry, which is well understood, and the constraint is drainage at the low point. A dip in a road is a place water collects, so the undulation has to coincide with a place that can be drained, and that is the detail that will decide where it can be used.
The honest limit is that this is a retrofit nowhere and a rule everywhere. It belongs in new corridors, written into the standard before the alignment is fixed.
Let the road bend. The walk should be flat.