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OK, here's a more detailed explanation:
A proper exposure to bring out Milky Way structure is going to be somewhere around 10-30 seconds at f/1.4-2 and ISO 1600-3200. Star trailing (as is just beginning to visible in the original image) occurs when the apparent rotation of the star field projected on your image sensor is more than a pixel or two over your exposure time. For me at 24mm with 6 um pixels this means that I can expose up to about 4-6 seconds without noticeable trailing. For other cameras/lenses multiply the 4-6s by (24mm / focal length) * (pixel size / 6 um) to get the max exposure time without trailing. Note though that this number is a worst case scenario for the western or eastern skies where apparent rotation is greatest. Shoot near the North Star (or the Southern Cross down under) and the trailing will be much less and exposures can be longer. Shooting near the northern or southern centers of rotation I often use 20s exposures at 24mm and show no trailing.
Now this pretty much sets your exposure for the sky. The foreground exposure will be controlled by whatever ambient light source is present. If the buildings in the OP's photo were faintly lit - flashlights, brief exposure to headlights, etc. - then the amount of light received by this sort of exposure should be about right to produce the image shown in a single exposure. If the buildings were well lit by outside lights, however, then this sort of exposure would strongly overexpose the buildings and a separate shorter exposure just for the buildings followed by blending of the two frames, as suggested by some of the other posters, would be necessary.
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