HRS 291E-61(a)(2) prohibits the operation of a motor vehicle while under the influence of any drug “that impairs the person’s ability to operate the vehicle in a careful and prudent manner”.  There is no exception to HRS 291E-61(a)(2) for medical marijuana.  Thus you can in fact be arrested for OVUII and your driver’s license can be administratively revoked if you operate a vehicle under the influence of marijuana – medical or not – if it impairs your ability to operate the vehicle in a careful and prudent manner.  Note also that HRS 329-122(c)(2) does not permit the use of medical marijuana in any moving vehicle, and HRS 329-122(c)(1) does not permit the use of medical marijuana “that endangers the health or well-being of another person”.