We’ve had a few variations of street writing robots at our Maker Faires and they always attract the attention of those near by. There are multitudes of ways to write on the road, but often the materials used can last longer than you want, or even be permanent. Instructables user Charles Glorioso shares a very clever version that uses pressurized water for his street messages.
He’s using a 3d printed manifold that is under pressure, with fuel injector solenoids to release the water. This is quite clever, as the solenoids can be purchased cheaply or even harvested from junk yards. They only take a simple pulse of 12v to release a jet of the liquid built up behind them. For those concerned about safety, the pressures involved here aren’t enough to cause an explosion with the 3d printed manifold, at worst, you’d just spring a leak.