A still image taken from police dash cam video allegedly shows Walter Scott running from his vehicle during a traffic stop before he was shot and killed by white police officer Michael Slager in North Charleston, South Carolina, April 7, 2015. Reuters/South Carolina Law Enforcement Division

Video footage from a witness was key in showing that Michael Slager, a white police officer in South Carolina, gunned down and killed Walter Scott, an unarmed black man, shooting him in the back as he ran. But when citizens aren’t around to start recording video at the right moment, many wonder whether wider use of dash cams, cameras mounted to the inside of a car windshield, could gather critical footage that could prevent police shootings or at least provide clearer evidence in the future.

A dash cam attaches to the inside of a car windshield, and advocates for its use point to the fact that it provides impartial evidence that can be particularly useful during insurance disputes and in court, especially when footage is clear-cut. But sometimes it fails to provide a complete picture, in part because such cameras record scenes from just one angle.

In the case of Walter Scott, the dash cam recorded interactions between Scott and Slager before the actual shooting, which took place out of view of the camera. “It’s very helpful, but it’s not conclusive evidence,” Scott Diamond, an attorney in Philadelphia, said, speaking generally about dash cam footage.

The community of Renton, Washington, began using cameras in police cars in 2010 and has credited those cameras for saving the city money and reducing liability claims.

In Cleveland, 18 seconds of audio from a dash cam currently play a key role in the trial of Michael Brelo, a police officer who was charged with manslaughter in the shooting deaths of two people, Timothy Russell and Malissa Williams, in 2012. Both appeared to be unarmed, and their deaths -- after 13 police officers fired 137 rounds -- were cast as being racially motivated. The audio sparked debate over who was shooting at what time.

Rod MacIver of Vermont used video from a dash cam to successfully challenge a ticket for running a red light. After a police officer pulled him over and wrote him the ticket, MacIver filed a complaint and pushed to see the video from the camera, which proved he had not run a red light.

Dash cams first came into use in Texas in the late 1980s, Break.com reports. But they hardly resembled the ones used today. Those early cameras used VHS cassettes and sat on tripods. Now that cameras have slimmed down in size, they’ve become far less obtrusive and are easier than ever to deploy in vehicles.

In the U.S., it’s not clear how many police departments or individuals use dash cams today. Interestingly, however, dash cams are widespread in Russia, Wired has reported.