Sometimes, you can't remove all of the bucking on a concrete road surface. It's due not so much to the spacing of the expansion joints, themselves, but to what is happening in between the joints. Heavy traffic will actually bend the slabs so that each expansion joint is at the end and beginning of a slight ramp. If the spacing of your axles is just right and at a certain speed, the harmonic bucking will be extreme.
About the only thing Hitler got right was the dimension of the Autobahn. There is no bucking on that freeway as the slab is 21"-30" thick, compared to our Interstate system which is 11" thick.