There's a separated pedestrian walkway in this tunnel, which offers some welcome shade as you walk up toward Kē'ahi (Diamond Head). Keep in mind that the tunnel and Diamond Head State Monument are only open from 6am-6pm. Authorities take that very seriously so if you stay after 6pm, you might be prosecuted for trespassing.
If you are not a Hawai'i resident with a Hawai'i ID, you need advance reservations to enter the monument. Make them online via this website gostateparks.hawaii.gov/diamondhead ; they cost $5 per person and, if you're driving, $10 per vehicle too.