A recently published series in the San Francisco Chronicle highlights the fact that ambulances are not reaching people fast enough. In the last four years, at least 439 people have died in San Francisco while waiting for ambulance crews who arrived after the city's target time. As the linked article points out, even a 30-second delay can mean everything to a patient who's waiting for life-support systems.
http://www.sfgate.com/cgi-bin/article.cgi?f=/c/a/2008/04/13/MNR0VTB9I.DTL