Though Siwa is only about 50km from the Libya, it’s currently illegal to cross the border in either direction, either along this stretch of border, or via the official border crossing in Sallum.
Siwa's bus stop and ticket office is opposite the tourist police station; when you arrive into town, however, you'll be let off near the central market square. It's sensible to buy your ticket ahead of time as buses are often full.
From the bus stop, West & Middle Delta Bus Co buses depart for Alexandria (LE75, eight hours), via Marsa Matruh (LE40, four hours) at 7am, 10am and 10pm. The 10pm service costs LE10 more. The daily Cairo-Siwa direct bus service had been temporarily suspended while we were in town, meaning travellers had to change buses in Alexandria. We were assured it would be back on the schedule by the time you read this; the cost should be LE130.
Microbuses going to Marsa Matruh (LE40) leave from the main square near the King Fuad Mosque. They are more frequent and way more comfortable than the West & Mid Delta bus, and the same price.
Construction of the Siwa–Bahariya road began in 2005 and, after many delays, is now finished. However, it was closed to foreigners at the time of writing.