Are you sure you have the legal right to put a pneumatic tube across the highway?
Results from anything you build yourself may not be admissible in court, particularly if you are not an expert at such things and do not adequately validate it before use. If you are intent on that approach, rent a certified unit.
As an alternative, I suggest having a group of residents approach the city. If you are the only one it bothers, you will have a hard, uphill battle. Again, the absence of your location is problematic. In my community, we had a similar problem with speeding. It happens that a police officer who works in another district also lives in the community. Working behind the scenes (officer to officer) he got speed monitoring put up -- not a camera, just one of those units that displays your speed. That was helpful.
In other words, cities have a lot of your money to fight against your "case," and if they need more, they will just ask you for it. The best approach is usually to use a little honey plus getting others to join you. It's called "local politics."
John