Levi loves to ride his bike, but I don't always want to chase him up and down the sidewalk while he does it. He already knows not to go in the road, but he was heading all the way down the sidewalk, where I couldn't see him. So I drew a STOP sign and a 'U-turn' arrow at the point where I wanted him to turn around. It took a few times of reminding him to stop there, but he got the idea, so now I can actually sit down and read a book for a while while he goes back and forth to his stop sign!
Other ideas for setting boundries:
Play 'red light, green light'. We play when walking around the block, and take turns calling out the 'lights' (but everyone stops when red light is called!). The idea being once the child understands what 'red light' means, you can yell that when they start running off in the mall,, crowd or by the street, and hopefully they will stop! With enough practice, it becomes an automatic reaction for them to stop, and saves you from running after them yelling 'stop stop stop'.
Visual boundries are a big help, in locations like parks and the zoo, I tell Levi 'don't go past the ____" (fence, Rhino's, edge of the mulch, etc.). That way he can SEE when he has gone too far (vs. telling him something like 'stay where I can see you')
Establish rules for crossing the street. Our rule is you stop, look both ways, listen, and wait for an adult to say 'OK'. Now he does this EVERY time we get anywhere near a road, even when I forget he will stop me!
Bike saftey vest - white and yellow, with reflection tape and glow in the dark paint for visibility, there is a a big reflective triangle on the back (like a 'caution' sign)
And a shirt I made in honer of Levi's favorite question to ask fellow bike riders!