I can close this if it's a dup but my Google-fu didn't find a satisfactory answer. In the middle of the floor of some older automatic transmission sedans, there is a bump (my old 4 dr 2005 Accord, 4 dr 1984 Volvo DL, 4 dr 2000 Saturn LS1). I'm not seeing this same bump in newer sedans (2016 Civic, 2009 Civic). I think all the cars I listed are FWD so that bump can't be for RWD. What is the bump for? Thanks in advance.
The hump provides longitudinal rigidity in the unibody. The fact that it also provides a place to route the exhaust is a bonus.
Notice that modern CAD-designed vehicles have much less of an obvious hump, unless they also have a roofless option (cabriolet, convertible, removable hardtop). On these vehicles the center hump is the main rigid structure in the vehicle, and is still pronounced.
Anyone can feel free to disagree with me since I'm not 100% on this, but I believe those bumps are placed into the design in order to give the exhaust system a little more breathing room.
Note the hump in the actual body of the car above the exhaust here. This is to aid in keeping unwanted heat away from the passenger area of the car while the heat shield provides an extra layer of protection on top of the physical space around the pipe.
I don't think the hump is there just for the exhaust, although as was pointed out in the comment it adds ground clearance which is a nice thing. The "transmission hump" (in honor of rear wheel drive cars) also adds structural strength – essentially a beam running down the center of the car.
I suspect that if it wasn't needed (or at least very useful) for structural reasons it wouldn't be there. Back when the hump was used for the transmission and drive shaft the exhaust managed for find its way to the back of the car without the hump…
The early SAABs didn't have a hump and it made for a much more comfortable interior (IMHO) especially in the rear seat.