This poster is presented with figure-ground and ambiguous figure of optical illusion. It encourages people making more use of public transit. You could see both passengers holding on to the hand grips on public transit and trees growing, which implies that an easy act that starts from daily life makes trees prosperous.
The slogan “Way to Go Green” has multiple meanings as well. Firstly, “way to go” is recognition of the act of the second part of sentence of going green. Secondly, passengers’ taking public transit shows that they are going somewhere green, which helps create a city forest. Take a ride, for yourself, and also for the trees!