'by roadside' is correct and usable in written English.
You can use it when you want to describe the location of an object or place. For example: I found a four-leaf clover by roadside.
Pakistani troop convoys often are hit by roadside bombs.
Water buffalo grazed by roadside thickets of bamboo.
One linked by roadside chats; the other by FaceTime.
The I/M program is supplemented by roadside checks.
Then, two Strykers were attacked by roadside bombs.
Highland Ave., Cayuga Heights, by roadside in shaded gully, 1913 (E. L. Palmer).
Also on Friday, three coalition service members were reported killed by roadside bombs in two attacks.
Commanders said three vehicles were hit by roadside bombs, though without causing serious injury.
We here aim at illustrating multiple ecosystem services that may be underpinned by roadside vegetation.
Fourteen marines from their battalion were killed Wednesday by roadside mines.
About half the deaths were caused by roadside bombs, which have grown larger and more sophisticated.
