23 C.F.R. § 1340.3
As used in this part-
Access ramp means the segment of a road that forms a cloverleaf or limited access interchange.
Cul-de-sac means the closed end of a road that forms a loop or turn-around.
Non-public road means a road on which members of the general public are not allowed to drive motor vehicles.
Nonresponse rate means, for any survey variable, the percentage of unknown values recorded for that variable.
Observation site means the physical location where survey data are collected.
Passenger motor vehicle means a motor vehicle with a gross vehicle weight rating of less than 10,000 pounds, including a passenger car, pickup truck, van, minivan or sport utility vehicle.
Service drive means the segment of a road that provides access to businesses and rest areas.
Traffic circle means the segment of a road or intersection of roads forming a roundabout.
Unnamed road means a road, public or private, that has no name or number designation and is often a farm or logging road.
Vehicular trail means a road designed or intended primarily for use by motor vehicles with four-wheel drive.
23 C.F.R. § 1340.3