I see some figs that many Mayor mod plops could be plopped on road.
I want to know how can I do that.
As far as I'm concerned, this is not possible (someone more experienced maybe can answer as to why this is), unless you mean underneath viaducts and elevated highways and such. Objects can appear on roads, but I think only as T21'ed props, right? But of course I'd be more than happy if someone proved me wrong.
It's not possible to plop MMPs on the road, or any other transit network for that matter. However some MMPs are of such size that if you plop them close to the road, the model will overhang a bit. For example the street in the upper right of this pic:
(https://dl.dropbox.com/s/ovb00670efu648r/SSW-1.png?dl=0)
I plopped Girafe's Oak trees very close to the street and since the models are quite large they extend to the other side, effectively covering the streets. The same goes for basically all MMPs, depending on their size of course. You'll have to play around a bit to discover which MMPs work best for whatever result you want. Also, the tree on the roundabout is an example of a T21'ed prop which Themistokles mentioned. Hope this helps. :)