More than 32,000 people are killed and 2 million are injured in motor vehicle crashes in the U.S. every year. A third of crash deaths involve drunk driving, and nearly a third involve speeding, according to the Centers for Disease Control and Prevention.

But some of the most publicized instances of police brutality have taken place during traffic stops. So how do you determine when a driving incident requires law enforcement?

You should call 911 if you witness a motorist posing a clear and immediate danger to yourself or other motorists or pedestrians, or if the driver has already inflicted harm, like hitting a person or object. If you’re driving, pull over to make the phone call safely. Do not try to chase them, but take note of the direction they went and any identifying information about the car or driver.

Clear signs of dangerous driving include erratic behavior like extreme swerving, weaving, persistent tailgating, irregular braking or accelerating, ignoring traffic signals, or very narrowly missing curbs, objects or people.

If a driver is relentlessly tailgating you, try to change lanes, let them pass and establish a safe distance between you. If the motorist continues to follow you, drive to your nearest police station and try to avoid making eye contact so as not to engage them and escalate the situation.

Knowing when not to call the police is much trickier.

“It’s just such a hard issue,” said Sarah Seo, a Columbia Law School professor and author of “Policing the Open Road: How Cars Transformed American Freedom.” “Maintaining safe roads for everybody is a very legitimate and important concern of the government. On the other hand, because we’re so dependent on cars in this country, the traffic stop is the number one most common type of police encounter, and it involves so much discretion on the part of police.”

Reckless driving is typically a criminal offense in the U.S., but different jurisdictions have various definitions for what counts as “reckless,” Seo said. “There is a lot of ambiguity, a lot of room in that word ‘reckless’ for an officer to exercise his or her discretion.”

“They can issue a citation, or they can just give a warning, letting someone go without any record, fine or consequence,” she explained. “Or the law might allow them to make an arrest, which can be part of that individual’s record, kick-start the criminal process for them and carry lifelong consequences. The fact that individual officers can exercise a broad range of discretion and have that extreme range of actions they can take means that implicit or explicit biases can come into their judgment about which course of action to take.”

Other traffic violations are not necessarily criminal offenses, but it’s not uncommon for routine traffic stops to lead to arrests.

And it’s important to note the racial dynamics at play with traffic stops in the U.S.

People of color, particularly Black people, experience well-documented disparities in getting pulled over, said Leah Shahum, founder and director of the Vision Zero Network, an advocacy group that has set a goal of eliminating traffic deaths and injuries. “This is obviously a serious problem that we need to address, including rethinking if or how police enforcement should have a role at all in traffic safety efforts.”

Traffic stops are the most common cause of interactions between police and the public, according to the Department of Justice, and research from the Stanford Open Policing Project shows that police are more likely to stop Black and Latinx drivers and more likely to search them during stops. There’s also evidence that the bar for being subject to a search is much higher for white people.

Many highly publicized instances of police brutality took place during traffic stops, including the killings of Walter Scott (allegedly pulled over for a broken taillight), Darrius Stewart (broken headlight), Samuel DuBose (missing front license plate), Maurice Gordon (speeding), and Philando Castile (suspicion of robbery). Sandra Bland died in custody after being pulled over for failing to signal when changing lanes.

Given this reality, it might be best to think twice before reporting someone for something small and non-life-threatening like mild speeding, failing to use a blinker or coming to an incomplete stop at a stop sign. In a fender bender situation, calling the police may not be necessary when you can handle the issue directly with the other motorist. If there’s no injury and the damage looks minor, you may want to call your insurance company first to see what they say about a police report, however.

Whether heavily armed police officers are the best way to deal with every traffic issue has come up for debate.

“Don’t use a hammer if you don’t need to pound a nail,” economist Alex Tabarrok wrote for the website Marginal Revolution in June. “The responsibility for handing out speeding tickets and citations should be handled by an unarmed agency. Put the safety patrol in bright yellow cars and have them carry a bit of extra gasoline and jumper cables to help stranded motorists as part of their job — make road safety nice.”

Tabarrok and others point to Highways England, which employs nonpolice traffic officers, as an example. These traffic patrollers don’t don’t have the authority to issue tickets, but there have been discussions of expanding their powers, and they can still pass information to the police as needed.

On a more local level, the city of Berkeley, California, recently voted to use unarmed civilians for traffic enforcement rather than police. The city is still working out the details, Mayor Jesse Arreguín said following the vote.

Others have suggested that additional speed and red light cameras could help enforce traffic laws and improve road safety without involving armed officers, but this technology has sparked much debate about surveillance and where the cameras are located.

Another helpful step may be shifting the financial incentives for law enforcement, as tickets for minor infractions can be big revenue drivers for local governments.

Shahum believes the key to traffic safety lies in proactive city planning and “self-enforcing” policies and roadway designs: “We need to invest in safe environments that promote safe movement rather than depend on reactive, punitive enforcement methods or unproven, victim-blaming education strategies.”