If you are up near Deadwood then Spearfish canyon is the closest for a good road ride.

An simple internet search brings up this: https://www.blackhillsbadlands.com/o...reation/biking

On one trip I was staying in Hill City and did a ride out/back to the Deerfield reservoir. Rolling hills and pretty low traffic there. Spearfish canyon and Needles highway would be bigger elevation changes, and likely to have more tourist traffic. So going earlier in the morning would be best.

Custer State park wildlife loop is fairly flatish and not that long. Plus there will be tourist cars going real slow on it. There are much better options.

You could also day trip to the northwest from there to the Devils tower region and probably find some low traffic roads.

There is also some excellent mountain biking in the whole region. The Centennial trail goes the length of the Black Hills region and can be tackled in sections. Also closer to Rapid City there are some trail systems with some real technical riding.