mobile app: speed gun 507$

i iam looking for an app i can use in a mobile that i install onto my letter box and that measures the velocity of passing cars. the doc should explain the task. in general i need to find a method how to automatically estimate car speed at road from one perspective. so i think that a pixel change of ie 5 points on a road surface would indicate a movement, and each of these 5 points has a known distance to each other, so pixel change gives time, and distance is known, so speed can be calculated. now the question is how you can activate a recording when via pixel change. works only during day (at first), background color fading (sunlight) must be taken into account by storing a new background color for each of the 5 pixels ie every minute. but a car to road should be a huge color change i guess. then you can take the video and store, and the app has to open the file right away and start the speed analysis. if the speed (time between pixel points) isnt small enough (limit setting needed) , the file is deleted. if you are interested, i have other larger tasks afterwards. you need to read the doc!