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Hi i was planning to make the same project and start to check options, but as 007 mentioned, we need at least 40-50m range sensor. also GPS needed. as the distance to next car depends on the speed. like if you are going by 50km/h, you need to keep minumum 25m's distance, but if you go 100km/h than you need to have 50m distance. which is calculated by the distance car takes in 2 seconds with its current speed.
Here you are, hope it helps
I made it with some changes and works perfect, thanks a lot for the information.I use Serial LCD with SDL/SCL connection to save connection pins, instead of second servo, i made a ramp in the food box.