I would like to say that you can see a certain landmark on a road at a certain distance from, say, the start of the road. Can I say “At mile/kilometer six on the road to town X, you will see Y.”?
If not what is the proper way of saying it? Should we say “At the sixth mile/kilometer on the road …”?
If you’re driving on an interstate highway in the US, there are mile-markers that tell you how far you are from the southern and/or western end of the highway in that state, so, if you’re driving in central Illinois, you can say “You’ll see a Ferris wheel on the south side of I-72 at about mile marker 98” (referring to a ride at an amusement park on the southwest side of Springfield, IL). But most other roads in the US don’t have mile-markers, so, if you were not on the Interstate, you would have to say something like “If you go south from Springfield on Chatham Road for two miles past where Chatham and Wabash Avenue cross, you’ll see a Ferris wheel. The turnoff for the amusement park is just south of where Chatham crosses over I-72.” You’d have to rely on your car’s odometer–or your own sense of distance–to tell you when you’ve gone two miles.
So, in a word, I would not usually recommend “At mile/kilometer six on the road to town X…” unless the road has mile- (or kilometer-) markings on it. I would recommend something like “Six miles along the road from Z to X, you’ll see Y”.