My botanical expertise is limited to a general botany course about 15 years ago, but I recall that ethylene functions as a plant hormone, stimulating growth and ripening. Ethylene is also found in vehicle exhaust. Could there be a connection?
No. We are looking for an explanation of why some plants grow by roadsides and (apparently) not elsewhere. The same plants that grow along roadsides also grow in disturbed areas away from roadsides. There is nothing unique about the roadside environment that explains the distribution of these plants. Instead roadsides are the disturbed habitat that is most common, or at least most visible, to most people, hence where these plants are mostly seen.
Roadways are a contiguous environment that covers a huge area. Any plant that survives in that environment can spread along that road, to other roads, and cover a continent. Other specific environments tend to be pockets, unconnected, and therefore isolated. The environment of the roadside is the least isolated of all environments on any particular continent. Its narrow suitability to plant growth limits the total number of species that will be able to adapt to it. Those that do will rapidly spread throughout it.
By the way, migratory birds, which once followed mountain ridges, and rivers as guides to migration, now follow large roads as well.
Tris
“What have you done to that cat? It looks half dead!” ~ Mrs. Erwin Schrodinger