The Onion Spring OHV Route is rated easy for vehicles with good clearance and 4-wheel drive. If you don't have 4-wheel drive, you can run up the road. As I was running down it, I met a pair of backpackers running up.
The road goes 5.5 miles from the Kaiser Pass Road, near the northwest end of Lake Tomas A Edison to a parking lot between two trailheads. The Margaret Lakes Trail #26E03
starts just before the parking lot and leads to Arch Rock
, Frog Lake, the Margaret Lakes. The Meadow Water trail #26E04 starts just beyond the parking lot and leads to Rock Creek, with other trails branching from there.
The backpackers parking lot at Lake Edison is 1/4th mile east of the Kaiser Pass/Onion Spring junction. So for running the Onion Spring road adds about 5.7 miles to the run. However, a regular car can drive up the Onion Springs road at least as far as the stables. If you are with other people, you could save 0.6 miles by having one person drop everyone else and all of the packs just beyond the pack station.
The road is not a very difficult run. Most of the climbing occurs between 0.6 and 1.7 miles. After that, there are gradual ups and downs until the final short climb to the trailheads. When I went down the road, on a hazy day in mid August, it was just a chore. However, I can imagine it being much more beautiful in the spring or on a clear day.
By mid season, the first good water is at 8900 feet on the Margaret Lakes Trail or less the 1/2 mile down the Meadow Water Trail