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Join Date: Sep 2002
Location: LAX/TPE
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None of the major rental car companies currently prohibit driving Saddle Rd - although you might want to double check Thrift and Dollar. Someone left their Thrifty contract in the lav on my flight back and I noticed it had so many disclaimers about this and that, I know never to rent from them.
If you drive Saddle Rd, the middle part is re-aligned and paved - but - the western side is still the old-fashioned two-lane, hilly, road which is subject to diverse drivers and blinding fog. The eastern side near Hilo is under construction, and subject to delays during the week.
Although not explicitly stated, all rental car companies bar driving the Mauna Kea access road to the summit because a portion is unpaved.