The phrase "on a roadside" is correct and usable in written English.
It simply means "next to or along the side of a road". For example: I found a stone frog on a roadside near my house.
The night before that, American soldiers had found the bodies of two Arab men on a roadside, one shot in the head and one decapitated.
At one point, he stopped and sat down on a roadside guard rail.
"You have to see it to Belize it" was scrawled across the T-shirts I saw hung on a roadside stall in Belize City a decade ago.
After being repeatedly assaulted, they were dumped on a roadside.
After five months, he was dropped on a roadside in Albania.
Avé and Kamen meet cute on a roadside, thumbs extended, both in dirty jeans, completely untethered.
In 2000, two Antioch College students were shot to death on a roadside in Costa Rica.
But he had not been deported and killed on a roadside, either.
His decapitated body was discovered by American soldiers on a roadside in Baghdad on Saturday.
A prominent human rights worker was snatched outside her apartment, killed and left on a roadside.
The police found them on a roadside in Harrisburg, Pa., where they had just finished dinner in their van.
