I agree with most people here that for Mad Max or Fallout style collection 28mm is better scale.
20mm scale is good rather for distopian projects, moreover I'm afraid that Aberrant developes new post apoc system too slow... ( fortunately there are Stan Johansen Miniatures aout there).
At the beginning of this year I started huge 15mm prject inspired by movies like "Children of Man" and other distopian fiction.
I have to say 15 mm develope very rapidly thise times there are at least 2 companies that produces and develope strictly Mad Max style post apoc ranges unfortunately with out realistic vachickles. Despite of lack of 1/87 interesting vechickles on the market there are lot of military and sci-fi machines form GZG, old Corw or QRF ( not metioning PEter Pig very generic modern collection). Additionaly some of Hot Wheels/Matchbox cars fits well with 15 minis in my opinion ( especially trucks and all kinds of lorries). I personally use muscle cars as some kind of fantastic huge cars ( it is interesting that they have seats nad driving wheels in apropriate size
Additionally thanks to companies like GZG or Peter Pig there are lot of separate heads to easy convert little miniatures.
28 mm scale is of course more popular and I like it ( most of my colection is int this scale) but proces are haeavy for me ( I mean especially decent military vechickles)...