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I think it is not always bad to remove the pedestrian from the street.
I'll use a really good example of this, that TP knows, the connection between the Eaton Centre and the old Simpson's building, it makes it way easier to cross the street there and does not seem to impact pedestrian traffic in that area. Actually there is also the option to cross under the street at that location. These other options have little/no impact on the set of lights that are also there and the amount of pedestrian traffic in this area.
Another example of pedestrian bridges is the Skywalk to the Skydome, much nicer to walk in an elevated, heated environment when going to a concert then along the street.
There is a lot of debate about the impact of the skywalks on the street level activity in downtown Spokane. They were part of a renaissance at one time, but then downtown seemed to fade again. You might want to talk to someone there. Also, of course, Minneapolis.
It depends on the streets that they're bridging. In Newark NJ, there's a hotel/office complex around the Amtrak station that really does suck the life out of the surrounding local streets. If the streets were uninviting before the pedestrian bridges were built, they are only more so after they were built.
Six-lane higways, however, don't have much pedestrian appeal to begin with. There is the possibility of sinking the highway into a tunnel --for cities that can afford it. There is an ongoing controversy at the WTC site in Manhattan to that effect. The towers had formerly been linked by a large enclosed pedestrian bridge to a massive office and residential complex on the other side of a highway. Planners see an opportunity to bring this connection to street level. Others see the plan as wasteful and overly disruptive to businesses and residents in the area.
And since I'm from NJ, I might as well talk about Atlantic City. Casinos there sprawl for several blocks, connected by bridges. Only the Boardwalk has any interest to the pedestrian. Turn a corner, and you're faced with towering blank facades and a barren, post-apocalyptic cityscape. Vistors driving into the city are funneled right into the casino parking decks.
Let's not forget Minneapolis' stepchild, St. Paul.
The idea of cennecting buildings has a number of advantages from the perspective of taking people out of the elements, and thereby encouraging walking. Many of these systems (St. Paul, for instance) have retail and services located on the level of the crossways, often extending the consumer space vertically, rather than horizontally. Smart growth, perhaps?
On the other hand, they do have drawbacks. Perhaps they siphon off some business from ground-level establishments. There are also security concerns, now that in the post 9-11 world many buildings do not permit people inside without going through screening. Lastly, some of them can be just plain ugly.
I haven't been able to find any specific resources regarding the impact of overhead pedestrian networks, but agree with some of the sentiments expressed here by others.
The answer in this case is a solid "It depends".
In Toronto's case, we don't have many overhead walkways. Instead, we've created a system of underground pedestrian passages which link most of the major downtown office buildings with Union Station. Most of the passages are lined with retail uses. It is quite busy during weekdays, but pretty dead on weekends. Some people argue that the PATH system has basically killed any opportunity for street-retail in the financial district, but I think it works quite well.
While there is a general predisposition against pedestrian bridges, for the reasons noted above (killing off street level life and retail, often tied to car parking directly adjacent to off-street development), there are some cases where it works.
It helps with getting folks across dangerous highway-like roads, but getting people to go up and over is hard. If the grade crossing is not sufficiently scary, they skip the bridge.
A pedestrian bridge can connect important existing above-grade nodes that are separated, usually by roads or transit. A good example in my town, Boston, is the bridge between Copley Place mall and the Prudential Center mixed use development. These two developments are period pieces that you wouldn't build today (isolated, auto oriented), but given their nature the ped bridge works well.
Pedestrian bridges can be useful in campus settings, allowing connection between academic areas that promote collaboration that might not take place in entirely spearate buildings.