I've always wondered why elevated pedestrian walkways aren't more common. Too expensive, no doubt, but the savings in travel time and lives from accidents seems worth it.
In the dense urban case, where "elevated pedestrian walkway" means "getting people away from cars", it means decimating foot traffic to stores and restaurants, ceding the area to automobiles. Generally regarded as horrible for dense urban environments.You see this more in urban planning a few decades back, and it tended to not help the surrounding area. If you look at the places where people want to live and travel to, you don't see elevated walkways. So it's fallen out of favor, with more emphasis on getting speeding cars away from dense areas.
The exceptions are things like the NYC Skyline, which is basically a pedestrian freeway between and through areas already dominated by pedestrian foot traffic and relatively light and slow vehicular traffic.
See the Calgary +15: http://en.wikipedia.org/wiki/%2B15
They get around the foot-traffic problem by basically bringing the commerce up as well- it's like a giant indoor mall on the second floor of interconnected buildings.
An interesting extreme case is the south-east side of Yokohama station, where they (stupidly) put in one the most insane messes of concentrated roadways I've ever seen [not just 8+ lane roads, but 4-5 vertical levels of roadways!]. The area also has extremely heavy pedestrian traffic, which essentially all happens underground or on elevated walkways; basically nobody walks at ground level any longer. This may sound horrible, but in fact it's pretty nice ... you really don't realize what a mess it is unless you look back from a distance.
In both cases, the retail has followed the pedestrians under or over the ground...
Reminds me of a picture of a (rather short, ten foot) escalator going up to a gym.
Finally, we have a pedestrian crossing with lights. It's less safe for the people, and more inconvenient for the cars, and all because not everyone behaves nicely.