This is what I ride in the city, Marin Muirwoods 29er, but mine is an older model: http://www.marinbikes.com/2012/bike_...Muirwoods_29er

It's not the fastest thing around, but it is basically a mountain bike with a rigid fork like back in the early-mid 90s. This bike is a like tank in both the good and bad way. The big 42mm tires flatten the road and but the steel frame is a bit heavier than aluminum bikes -- about 30lbs. Despite that, the bike isn't much like a lumbering truck, it's reasonably quick and handles pretty well on the street but the handlebars are around 680mm so they are sometimes awkward trying to squeeze between cars. The tires don't have knobs on them so they do roll pretty easily. More than anything, the riding position and resulting aerodynamic drag will be the limiting factor. In San Francisco, I never even use the big chainring. Rack mounts front and back for any cargo you might want to carry.

I've managed to get my Muirwoods up to just under 40MPH on steep downhill street. The bike was stable, and it was a thrilling ride, but I was way out of my league -- up until that point I'm not sure I had ever exceeded 20, maybe 25 MPH riding around town.

The bike my housemate has is a Marin Point Reyes 29er, but also an older model. This one is aluminum frame with carbon fork. It's no longer made, but if you can find one on clearance, it's quite a nice ride. I think it's a bit zippier than my Muirwoods 29er without sacrificing comfort. http://www.marinbikes.com/2011/bike_...int_Reyes_29er

Rack mounts front and rear also, but I'm not sure I really trust the mounts on the carbon fork.

Both bikes are hybrids but they are closer to mountain bikes than road bikes. The fork and frames also have enough clearance than even 50mm tires should be ok if you ever choose to go that route. Compared to a Scott SUB something I once rode, these two bikes are like riding in a Camry or Corolla because they still bounce around a bit. That Scott bike with aluminum frame and rigid aluminum fork was like riding in a sports car with super stiff suspension because it was throwing me around. That Scott SUB was unacceptable for me if I were to ride it daily in the city since roads here are quite beat up. I don't know how a 75 Cadillac rides, but if you want something like a Crown Vic taxi, I would say that's more like one of those 26" comfort bikes that are upright and slow with fat 26x2" street tires and suspension.