The Guide “10 Tips For Best Free Flow ANPR Deployment” aims to provide a useful list of characteristics and features of ANPR cameras to be used in free-flow, auto-triggered and high-speed traffic control applications.
Capturing the vehicle and an image of the license plate in an exact and deterministic point of the road is essential to guarantee the target resolution of 2,7 pixels/cm for the license plate, even when a megapixel sensor is used (see Tip #1).
A great number of auto-triggered ANPR cameras use image sensors at low to medium frame rate speeds: capturing between 15 to 30 images per second (or FPS, frames per second) in a field of view of around 5 to 10 meters (virtual detection loop). This has at least one important consequence for the ANPR performance: the resolution of the license plate image decreases as the vehicle travels away from the camera. If the vehicle’s plate is captured at the shortest distance of the camera’s field of view, then maximum resolution of the license plate image will be attained. On the contrary, if the vehicle is detected when it is at the limit of the virtual loop (longest distance from the camera), then the resolution of the plate will be lower (less pixels per centimetre).
An OCR engine will underperform on a plate image with low resolution (as explained in Tip #1).
Furthermore, a non precise auto-triggered ANPR camera will impact the precision of a free-flow travel time or average speed application. There would be no certainty on where (position, km point) the vehicle is captured and thus it would create an error on the speed measurement.
Instead of working on a virtual detection loop in a range of 5 to 10 meters, Imagsa's ANPR cameras define a virtual loop in the lowest part of the overview image (a region of interest of 1,5m - see video). This is the closest point to the camera. In this region, 250 images per second are captured and analysed in real-time thanks to the power and intelligence of the Mercury’Traffic ultra-fast smart camera. Thus, the vehicles’ plates are always detected at the same point, with the same target resolution of 2,7 pix/cm. An optimum plate resolution and a good illumination of the plate image ensure that the OCR engine always achieves high recognition rates.
Additionally, Atalaya’s highly precise auto-trigger is key to achieve great precision on a free-flow (no externally triggered) average speed enforcement solution.
C/ Sancho de Avila 83-89 // 08018 Barcelona // Spain // +34 934 869 345
