Who says there won't be an underpass/overpass connected to the single-purpose airport access road? There certainly will be.
What the state Department of Transportation isn't saying is that it plans to build the overpass/underpass at the Old Haleakala Highway intersection (behind Costco) and not at the Hana Highway intersection. So, instead of serving the local community's transportation needs of 5,000 to 10,000 cars per day, it will serve visitor needs with 500 cars per day.
But not to worry, a red light at the Hana Highway intersection means different things to different people, so what's the difference?