A spot where you can't parkit means there is an obstruction and should be treated as red lines
Roadside signs
Yellow, diamond warning signs.
To warn drivers of their presence and to pull people over with.... Do you seriously have to ask?
The car horn should only be used to warn other drivers of your presence.
to warn the drivers behind your automobile that you are slowing down
Warn drivers about what's ahead.
Red and
New
Both of the following, To keep track of drivers who get too many tickets. To warn, suspend or revoke licenses for bad drivers.
When driving in the mountains, you will often see signs along the street that warn drivers about falling rocks.
Paul Revere rode to warn the people of Lexington and Concord that the British were coming. However he was captured. William Dawes was the rider that made it through the lines.
That is to warn birds so that large flock of birds do not fly into the power lines. Also helicopters.