They are also designed to park "end-on" to the curb - they are the length of a normal car's width. Great for those San Fransisco hills?
Only downside is there's not much room between your arm and the window. They are generally very safe, but a friend's friend (sorry) toppled one on a motorway, slid it on its side and mashed his arm nastily. I'd still get one for the city driving tho'.