I’m not sure where I first found this diagram before, but I found it again via Frumin and decided to reformat it for my own use. This diagram comes from Urban Transit: Operations, Planning and Economics (see update below) by Vukan R. Vuchic a Professor of City and Regional Planning at the University of Pennsylvania.
Areas Required for Transporting 15,000 Persons per hour by Different Modes (Vuchic 1981):

After the jump are the individual stills and I also complied all of thje information into one PDF: Areas Required for Transporting 15,000 Persons per hour by Different Modes (Vuchic 1981).
Update 28 May 2008
Dr. Vuchic writes in that the figure represented is actually from Urban Public Transportation Systems and Technology (Prentice-Hall 1981) but is available in Urban Transit Systems and Technology (John Wiley & Sons, 2007) which is a new edition of the work. Dr. Vuchic also notes that Urban Transit Operations, Planning and Economics (John Wiley & Sons, 2005) and his third book of my Transit Trilogy Transportation for Livable Cities (CUPR, Rutgers University, 1999) are two other good sources for transportation policy and economic study.