Go early. If you arrive before 8am, you don't need a reservation. However, you still need to pay for the entry fee per car unless you have a valid park pass or Nation park membership.
If you go for sympathy to Red Rock they open at 8 a.m. only but I go on a hike along Highway 159 they have about five different ones and it's open 24/7 no emission needed Tri Oak Trail one of my favorites
We just went and the agent made us a reservation on the spot.
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