One thing that can be certain is that standing or sitting next to a literature cart is not as effective as calling onto people's doors. People are are walking down a street for a reason , most of the time religion is not on their minds.
Further to that JWs are perceived as nuisances, religion pushers that disturb people usually in the morning at their front door..
People are a little more hesitant to talk to people openly in the public about religion right on a public street anyways.
The hours that pioneers accumulate may be easier as far as foot work and perhaps gas and wear and tear onto vehicles but the effectiveness is not as strong as going directly to people's doors.
In the endeavor to sustain a religion in devoted followers/numbers, it would assumable that street vending is not as productive as calling onto people's doors and this connects to the financial sustainability of the organization itself if the numbers fall or decline.
From a person who has been directly and indirectly involved with this organization, I'm actually surprised that this cart witnessing has taken such a strong presence with JWS in most modernized countries.