Around the time of the GREAAAAATTTT SKYBUS, there an additional piece of the Pittsburgh Transportation Study was going on to plan a modern highway beltway system throughout Allegheny County. Presumably, this would go hand in hand with a rapid transit system to help catapult Pittsburgh into the 21st century.
If you look at the original plan, you'll see an extensive network of roads throughout the county. I'll be here all day, and you won't waste your time reading this if I were to detail every highway that was to be part of this system, but suffice to say that the proposed system looked like a plate of spaghetti on the map.
If at least part of this would have gone through, in conjunction with some form of rapid transit system, whether it be skybus or subway, Pittsburgh would have ended up with a pretty comprehensive transportation system providing the city and county with multiple mode options to residents of the area.
The one key difference between this failure and other past and present transportation failures in Pittsburgh, is that this one was not actually due to political wrangling in local government. This is not to say that certain factions didn't oppose portions of the proposed highway system (WRATH, a collection of citizens from Whitehall vigorously fought a south hills expressway) , but it is safe to say the plan wasn't killed from local government action, or lack thereof.
This time it was the state of Pennsylvania that did this project in. As of 1969, it seemed as though everything was on track for this new system. Land acquisition had already begun on several of the projects.
However, by 1973, all the projects were dead. I did some researching (I use that word loosely) to try and find out exactly why funding fell apart. I was unable to find out, but I do know that the plug was pulled on each individual route, and they were all cut at about the same time. Click here for a pretty good list and some interesting information about each of the planned highways and information about their planning and ultimate demise.
Unlike with other projects, this time the demise was so complete, that no part of the system was built, not one highway. The only structure Pittsburgh gained was the Birmingham Bridge. It is only piece of a "Oakland-Crosstown Freeway", and of the entire beltway project ever built. The Birmingham Bridge joins other notable "achievements" in Pittsburgh as the Wabash Tunnel and the Pittsburgh and Steubenville Railroad Tunnel as monuments to failure.