There would be less light at the road level but there is no side barrier between it and the street. A bit dimmer than under a gas station island — which is lit from all 4 sides instead of 1. Similarly on the elevated way, there is either only glass or just a safety fence on one side. It would be better light level than the edge of a parking garage. If you want greater light levels, use translucent roofing panels. Or paint the concrete white.
Going up and down only happens when you enter/exit the downtown grid. Every pedway crosses each intersection — so in a fully developed pedway system, there are 4 pedways across each intersection. I know that as a cyclist, given the choice of climbing a ramp, or biking a mile of downtown traffic, I know which I would prefer.