If you are reporting a non-emergency traffic violation, you don't want to call 911, since 911 is reserved for true emergencies. Many cities have a special service for non-emergencies, where you can call 311 to make a report. Some cities also allow you to report a non-emergency traffic violation through a city website. In cities without 311 service, call the non-emergency police number to report a traffic violation, and have all the information that you need to report written down for quick reference during your call.
TL;DR (Too Long; Didn't Read)
Many cities allow you to dial 311 to report non-emergency situations, including traffic violations. If your city doesn't have 311, you can also call the local precinct's non-emergency line.
Reporting a Non-Emergency Traffic Violation
When you call to make a report of a non-emergency traffic violation, you should be ready to provide the date and time of the violation, the location of the violation and as much information you have about the vehicle and driver involved in the violation. If the violation involved an accident, it is prudent to contact the police before you leave the scene even if there are no injuries or obvious damage to the vehicle.
Cities with 311 Service
Baltimore was the first city to create a special number to call when you needed to make a non-emergency report of a traffic violation or to get further information. Like 911 service, 311 service connects you directly to an operator who can take your traffic violation report. Philadelphia, Chicago and New York also have 311 service. Some cities even have a 311 app, which you can use to report non-emergency issues such as potholes, traffic violations and building code violations.
Cities Without 311 Service
In those thousands of cities that do not have 311 service, the easiest way to report a non-emergency traffic violation is to call the non-emergency police number. You can locate that number in the front of your phone book, or by doing an internet search for "non-emergency police number [your city]" to locate the correct phone number.
911 for Reporting Safety Issues
If no one is available to assist you at the non-emergency number, you can call 911 if you think the traffic violation may be a threat to someone's safety or property. If you see someone double parking, for example, 911 is probably not appropriate, but if someone is speeding excessively or driving in a reckless manner and may hurt someone, 911 might be the right approach.