The answer to your question varies slightly depending on what technology the officer used to obtain your speed. In Maryland, almost all stops for speed are based on either LIDAR or RADAR (although your speed can be determined by pace, VASCAR, or other means). How the unit works and how the officer determines that the speed reading is from your vehicle differs slightly between the two.
LIDAR units use pulses of light to determine the speed of a vehicle. When the officer lines up the sight on the LIDAR unit with the front of the car, he/she pulls the trigger and three pulses of light (laser beams) are shot at the target, reflect off of the target, and are then 'received' by the LIDAR unit. The unit then uses distance the laser beams traveled, along with how long it took them to travel the distance to determine the speed of the target vehicle. LIDAR units are typically certified to be accurate to +/- 1 MPH and are claimed to be vehicle-specific to up to 4,000 feet or 3/4 of a mile, although most readings I see are under 2,000 feet.
LIDAR units are deemed to be vehicle-specific because the 'angle of influence' (how wide the beam is) of the laser beam is so narrow that even at a distance of 4,000 feet, the beam should not be wider than one car.
RADAR units use pulses of radio waves to determine the speed of a vehicle. Unlike LIDAR units, most RADAR units are always on, sending and receiving signals (although there are some instant-on radar units). RADAR units also have a much wider 'angle of influence', so the officer must take additional steps to make sure that the reading the unit is displaying is accurate and for the proper vehicle. As part of the 'proper operating procedures', the officer is supposed to visually estimate the speed of the target vehicle. The reading on the RADAR unit should correlate with that visual estimate.
Pacing is when the officer essentially follows a vehicle and uses the police cruiser's speedometer to get a speed. In theory, when a police officer paces a vehicle, the officer's cruiser and the target vehicle are traveling at identical speed, with the target vehicle immediately in front.
VASCAR is used in a manner similar to a pace, but the police cruiser does not need to be traveling at the same speed as the target vehicle. In fact, the police cruiser can be stationary. VASCAR works based on the mathematical formula of distance divided by time equals speed (e.g. 65 miles divided by 1 hour = 65 MPH). When stationary, the officer measures a distance between two objects (for example two telephone poles) and enters that into the VASCAR unit. When a vehicle passes the 1st known object, the officer clicks a button starting the timer on the VASCAR unit. When the vehicle passes the 2nd known object, the officer clicks the button again, stopping the timer. The VASCAR unit then calculates the average speed of the vehicle between the two objects.
Regardless of the method the officer used to obtain your speed, an experienced traffic defense lawyer knows how to exploit every possible weakness in the State’s case. If you received a speeding ticket in Maryland, and would like to discuss your case with Scott, complete the Traffic Ticket Evaluation Form.