Typically 5%, but that would depend on your code.

2-3% is ideal, 5% max.

we did a bldg in evanston, il that had a 5.5% slope. I'm not really sure there is a 'code' for this (that i've found) other than what applies to any drive ramp, but it is my understanding that at when you get much steeper than 5-6% doors are gonna be likely to start closing on people, so its kind of a design decision based on where it is located in terms of density and what kind of use(s) it's serving (that is, how much loading and unloading into cars can be expected)...and remember that there should be a flat area for accessible stalls.

thanks for all the input

probably should check zoning code. 5% is max. for mine.

Check Graphic Standards. It has a pretty good section on parking from a trusted source. work for idle hands is right; the gravity pull on open car doors is the driving factor for maximum ramp slopes.