Why didn't I get this SAT writing question right?

xxeliza321xx
Wed, 29 Aug 2012 15:31:58 +0000

Sentence: In those cities in which public transportation is adequate, fewer traffic problems occur and pedestrians are rarely involved in accidents.

Underlined Portions: A-those cities, B-in which, C-are rarely, D-involved in, E-no error

The correct choice is E, but I put B because I thought it should be ‘‘where’’…‘‘In those cities where…’’. Why is ‘‘where’’ wrong here?