Wave down
Definition 1 of 1
Pro Tip 1/3Visibility Matters
Use this phrase when you need to be seen by someone in a vehicle, especially in urgent situations. 
She had to wave down the ambulance from the side of the road.
Pro Tip 2/3Physical Movement
The phrase involves a physical gesture of moving your arm, often energetically, to catch attention. 
In a crowded street, he had to energetically wave down a cab to be noticed.
Pro Tip 3/3Seeking Assistance
It's commonly used to describe an attempt to get help when you're stranded or in need. 
Stranded after the hike, they waved down a passing car for a lift.