[insane_dreamer]

I'd say the barrier in N.America is both deficient networks in most cities, and price (it's actually quite expensive because there's such low usage). If it were free, or close-to-free, it would ostensibly increase ridership which would increase the impetus for further expansion (which right now is hard to advocate for when so few people use it; it's just not worth the cost even if you charge people for it).