Driving to Libya
You can cross into Libya by road from Cairo (12 hour drive to Benghazi) and from Tunis (11 hour drive to Tripoli) although visa formalities mean that travelling by air is simpler. Neither border post is particularly likely to be closed to foreign travellers, but if you have been granted a visa, it's wise to check if the means of travel is stipulated, before embarking on a journey by land (if your visa says you are travelling by air, you must.) Libya's land borders with Algeria, Niger, Sudan and Chad are closed to non-Africans.
From Cairo, you will need to drive west, crossing the border into Libya near Al Burdi. This was the entry point for the majority of international journalists covering the conflict in the eastern part of Libya. The border post still houses some of the people displaced by the Libyan conflict, and can be chaotic. Expect to spend about three hours crossing the border. You will need to get your passport stamped on the Egyptian side and again on the Libyan side, and you may need to show a yellow fever certificate if you have recently travelled to an affected area. There is a short distance to walk between the two border posts; if you have lots of luggage you might wish to pay a porter to help carry your things. Once on the Libyan side, it's about a further six-hour drive to reach Benghazi.
From Tunis, you can take minibuses all the way to the Ras Ajdir border crossing, which lies about 11 hours' east of Tunis. If you're traveling with your own car, the border guards may charge you a fee. In 2010, this was in the realm of 150 Libyan dinars per car. Reports from travellers who have crossed the border since the end of the conflict suggest that it is still possible, but that transiting through Ras Ajdir can be slow due to increased formalities. You can also enter via the Dehiba/Waziin border crossing, further south.