Recently, the Rabbinical Committee of Transportation (whoever they are) distributed a brochure in the Bais Ya'akov system advising girls to ride in the back of the bus (as is done on mehadrin buses) so that riding can be done in a "halachic" manner.
Of course, the interesting thing about all this is that it has nothing to do with halacha. There is no halacha anywhere in Shulchan Aruch or anywhere else that says that men and women cannot sit together when riding in public transportation. Were it so, then thousands (millions, maybe?) of Jews would be violating halacha every time they boarded the New York City subway system or rode on a city bus.
Call it a chumra. Heck, call it a hiddur if you like. But don't suggest that thousands and thousands of people who are shomrei Torah U'Mitzvos are violating halacha every day by suggesting that complete segregation is the only "halachic" way to ride the bus.