Blowing the whistle on bad drivers can be tricky

Then he sped off.

“The guy in the car was shocked, as was I,’’ said Harding, of Westwood. “I felt I had no recourse because I didn’t get the plate number. What could I - or the guy whose car got hit - have done?’’

Bad driving is something most of us encounter every day, given the level of aggressiveness on Massachusetts roads. And when another motorist’s dangerous actions put you or your loved ones in danger, you want the jerk to get caught.

But here’s the question we’ve all asked ourselves: Will your call to the police really make a difference?

And would a cellphone video of the culprit help?

First things first. To report reckless driving, dial 911 any time, anywhere, police say. If you’re on a cellphone you will automatically be connected to the State Police. They can either take your complaint or patch you through to the local police department.

Dialing “*SP’’ on your cellphone - the letters standing for “state police’’ - no longer works, police say. Always dial 911.

Naturally, the more details you can provide, the better: the make, model, and license plate of the offender’s vehicle, where the reckless driving occurred, and any other information available. But even with a thorough report, police might be limited in what they can do.

If the violation you witnessed was “civil’’ in nature, such as speeding, running a red light, driving without headlights, or parking in a handicapped spot, the police’s hands are tied. In order to cite someone for a civil moving violation, an officer of the law must see the violation firsthand, said David Procopio, spokesman for the Massachusetts State Police.

The policy, while frustrating, safeguards against false accusations, particularly in road-rage cases where a driver may want revenge, Procopio said.

However, if the violation you witnessed was serious enough to be considered a criminal violation, the police can pull over the alleged offender.

Driving recklessly, driving under the influence of drugs or alcohol, and driving to endanger are all criminal offenses.

“Say we get a call of a gray, 2000 Honda Civic weaving between lanes. The guy is driving like he may be drunk,’’ Procopio said. “A police officer encountering that vehicle 10 or 20 miles up the highway would have reasonable suspicion of criminal activity to stop that vehicle even if it is no longer engaging in that behavior.’’

In that situation, the officer couldn’t ticket or arrest the driver based on what you saw. But, having pulled over the alleged offender, the officer has the opportunity to look for something that he or she can investigate. Is there an open container of alcohol in the car? Does the driver’s breath smell of alcohol? Is his speech slurred?

There is one important caveat: The police can only stop another vehicle if you provide your name and phone number to back up your statement. And if you do provide your name and address, you could potentially be called as a witness if the offender is charged and the case goes to trial.

“It’s the call that established the reasonable suspicion that allows us to stop the vehicle, so we have to include that in our reports, and that is discoverable by the defendant in the trial process,’’ Procopio said. “The defendant’s lawyer would have access to that report and the person could be called and his observations challenged by the defense lawyer.’’

The reason for this is the Sixth Amendment to the Constitution, which guarantees defendants the right to face their accusers.

The policy affects police nationwide: Earlier this week, the US Supreme Court rejected an appeal by Virginia authorities who were forced to drop charges against a drunk driver who was arrested on the basis of an anonymous phone call.

If you don’t give your name, police can still be on the lookout for the alleged violator. “We’d still like to know,’’ Procopio said. But they can’t pull anyone over unless they see someone breaking the law.

The Registry of Motor Vehicles used to have a similar complaint procedure called the Who’s Operating Program. If you witnessed any improper driving, be it someone running a stop sign or someone driving drunk, you were able to fill out an online complaint form that required your contact information.

Under the Registry’s program, both you and the alleged offender were automatically called to testify at a hearing regardless of the complaint. The program was discontinued last year, though, because an overwhelming number of witnesses failed to show up at hearings, said Registry spokeswoman Ann Dufresne.

“It got to the point where it wasn’t cost-effective,’’ she said.

Local police abide by the same rules as the State Police, but your hometown might offer additional ways to file a complaint. The Lynn Police Department, for instance, allows people to report online everything from reckless driving to people who regularly speed through the neighborhood. But again, the complaints can’t be anonymous.

As for shooting videos of bad drivers, well, it’s probably more trouble than it’s worth, Procopio said.

“We obviously don’t want motorists shooting footage while they’re driving,’’ he said. “But if a passenger shot the footage, it would have to meet some very important conditions in order for us to be able to use it as evidence. And it’s unlikely that a cellphone would have the capability to show the type of detail we need.’’

For a video to be useful to police as evidence, it would need to include close-ups of the car, its license plate, and, ideally, the driver so he or she could be identified. But it would also need to show zoomed-out views of the roadway, with clear footage of the actual driving violation.

If a video showed all this, “It’s possible we could issue a criminal summons based on it,’’ Procopio said.

But the reality is that most people don’t drive with high-tech cameras in their hands, ready to grab video at a moment’s notice. And, as Procopio said, cellphone cameras almost always lack the range and clarity needed to provide such evidence. But who knows? In a few years, with technological advances, maybe that will change.

Somerville resident Peter DeMarco writes the column Who taught YOU to drive? for the Globe’s regional sections and updates a Facebook page of the same name. He can be reached at demarco@globe.com.