A: Grade Separated (Elevated/Underground)
B: Separated (Exclusive use, separated by vertical barrier)
C: Dedicated (Exclusive use, separated by painted stripe)
D: Mixed Traffic
This applies for all vehicles: Cars, trains, and bikes. The names change by mode, but the effect is constant: The higher the 'degree', the faster and more safely you can travel; less interference with other modes, less danger of another vehicle intersecting your path.
CARS
A: Freeway
B: Best car pool lanes
C: Most car-pool lanes
D: Most roads
TRAINS
A: Subway, (& best light rail)
B: Light rail (inter-urban configuration)
C: Light-rail, in-street for fire-lane
D: Streetcar, or tram.
Bicycle
A: Elevated bikeway
B: Separated bike lanes (bikeways)
C: Most bike lanes
D: Biking in the street.
A bike does not need a road. A road is engineered to support thousands of pounds per square inch. Something much lighter (and cheaper) works. But lets get this out of the way: Sidewalks are for pedestrians, not bicycles. Bikes travel at 5-15 miles per hour, three times of a pedestrian. The geometric design, in terms of clearances, straightness, and pavement quality that works for a pedestrian doesn't cut it for a bike. So we need a bikeway. Bikeways get called 'multi-use paths' so that people can also skateboard, rollerblade, and jog on them. It's a legacy of their historic recreational use.