Tell it like it is
The NCO Spring Ahead, in Naperville IL, does an excellent job of explaining the details of each of its races.
The event includes a 10k Trail Run, 5k Street Run (yes, that's what it's called), and 5k Wheelchair race.
Trail Run information is very specific. "The first and last 1.5 miles of the route are on the street," the entry form explains. It goes on to say the trail portion of the course includes dirt, mud, grass, and crushed limestone. "Expect exposed tree roots and other natural obstacles along the route."
Calling the 5k a "Street Run" succinctly differentiates it from the Trail Run.
It's best to give runners as much information beforehand as possible. They'll be better prepared to race and less likely to call you with questions that could have been answered in the entry form.