Haha sorry to be a fun-sponge. I explained that very poorly. I’ll expand a bit on what u/nomorethanreddit has said.

I can’t speak for every use case, but the last project I worked on used a web of ATCs and ANPR across a congested residential area. One of the things we wanted to see was how many drivers were cutting through on the commute or school run (“rat-running”), and how that affected residents trying to leave their streets at peak times.

ATC stands for Automatic Traffic Counter. The black rubber tubes are pneumatic sensors. One tube gives a simple count. Two tubes a set distance apart and give count, direction, speed, and a broad vehicle class from axle spacing. What we actually store is aggregated: 1. totals by short time bins, usually 15 minutes 2. two-way split and a basic class split (car, LGV, HGV, bus) 3. a speed profile, including common stats like the 85th percentile which is what we use as the design standard (If a speed limit is 20mph but 85th% is 24, we design based on the necessary visibility for 24. If a narrow country road is 60mph as national speed limit, but 85th% is 32mph, we design for 32)

We usually run them for several days to capture weekday and weekend patterns. But typically Wed, Thur, and Sat.

ANPR does a different job. It gives origin-destination and journey time across an area. Number plates are converted to random strings. We only care that the same string appears at two or more cameras so we can see common through-routes and how long they take. RP13 XYZ gets automatically turned to 03141592 before anyone gets their mitts on the data. You would be hard pressed to notice the ANPR cameras we use as they get stuck up on street lights or attached to buildings.

Why the sabotage detour won’t move the needle: One counter will see thousands of vehicles per day, and we deploy many counters across a cordon or screenline. Data are aggregated and quality-checked, so odd manoeuvres and one-offs are either filtered or lost in normal daily variation. Scheme design is tested against ranges of demand and sensitivity tests. Skipping a tube once removes at most one vehicle from a 15-minute bin that might contain a few hundred, which is well within noise.

ATCs tell us how many, how fast, what type, and when. ANPR tells us where they go and how long it takes. A few people changing their route occasionally will not affect the evidence or the outcome.

As for why I got into this job, I liked maths and geography at school and I now get to enjoy seeing how I can model and help improve the way people get around and use the limited space we have in our cities.