Good Question. Not so easy to answer though. Hawaii Revised Statue (HRS) 291-21.5 is state vehicle law which applies to window tinting. You can view it at http://www.capitol.hawaii.gov/hrscurrent/vol05_ch0261-0319/hrs0291/hrs_0291-0021_0005.htm
Reading the following, sounds quite challenging to determine what is legal without some sophisticated measuring device, but someone in county government must have access to verify legality of County Vehicles. “Sun screening devices for side windows necessary for driving visibility which are to the rear of the driver and for rear windows necessary for driving visibility which, when used in conjunction with the glazing material, have a light transmittance of no less than thirty-five per cent plus or minus six per cent”
But to a citizen observer looks almost impossible to see anything inside the passenger compartment, and begs another question. Why is such dark tint needed on a Maui County vehicle as show here, legal or not?