This image shows a scene of a to-be accident by the couple, of a child. The rearview mirror reflects the eyes of the child that possibly could be theirs with the body of the child that was going to be hit.
I think the posters is trying to say that before thinking that it isn't terrible to hit a stranger (in this case, school boy), think how you would feel if it were someone related to you (in this case, your child). "Think of both sides" suggests us to think of the pedestrian ahead as someone of our own and ask ourselves if we would still take the risk of hitting the pedestrian.
They wanted to keep text minimal because in this poster, it's the picture putting most of the message across and that is what attracts most of the attention. However, they included the text "think of both sides" which suggests us to think of the pedestrian ahead as someone of our own and ask ourselves if we would still take the risk of hitting the pedestrian.
The "both sides" in "think of both sides" is coloured in a colour which stands out from the rest of the picture which gets people to think as the pedestrian as someone that means something to us (both sides- who is sitting at the back as the pedestrian walking ahead).
The text is placed on the top right as the right side grabs attention. However, it's size is not big compared to the rest of the poster because the picture depicted is what contains most of the message.
The colour of the poster,gray, is rather dull and gloomy because they are associating accidents with death which is not a laughing-matter. They also made the important parts of the text stand out by using a contrasting colour.
Purpose: To think before hitting someone (even accidentally) to prevent accidents
Audience: Everyone who drives
Context: Could happen anytime, anywhere when driving or we, as children, may even witness this as we sit in our parents car.
Culture: It is set in the present where most people drive and have cars