Flag down
Definition 1 of 1
Pro Tip 1/3Visibility Matters
When you try to flag someone down, make sure you're visible to them. 
He stood under the streetlight to flag down a taxi.
Pro Tip 2/3Safety First
Never flag down vehicles from the middle of the road; it's dangerous. 
She walked to the sidewalk to safely flag down a bus.
Pro Tip 3/3Urgency Implied
Using 'flag down' can imply an urgent need for the vehicle to stop. 
After his car broke down, he needed to flag down help.