I also think it is quite clever. It has been done at zero cost to the Society , who charge congregations for the carts. And by ensuring only "qualified" JWs have the "privilege" of manning the carts , it confers a fake prestige ( and very easy hours to the pioneers who mainly look after them - I have heard that they also can count the time transporting the carts to and from the location. )
And with the "door to door" work pretty invisible these days , it does present a tangible presence - even if hardly anybody takes literature.
I noted a couple of observations from the Guardian article below that relate very much to my own experiences with the carts. The writer observed that they asked for a pamphlet but the JW "sister" made no effort whatsoever to engage in any conversation - not even a simple question. And also that he watched them for an hour outside a very busy railway station and the only people that went up to them during this time were people asking for directions. It seems a very passive form of "witnessing" , if , indeed , it can be called "witnessing" at all. I kind of can't really imagine Paul and Barnabas hanging around a cart at one of the city gates , scratching their asses as people walked past until their "hour" was up and they could skulk off back home,