[fps]

It's to notify police to stop the car if they see it on the road. The US has a lot of infrequently used or unused cars that gradually transition to a sedentary life in a garage or yard. It's not illegal to let your registration or inspection lapse if the car isn't being driven. If you drive a car with a lapsed registration or safety inspection sticker, police will notice, stop you, and issue a ticket.

[dmckeon]

> It's not illegal to let your registration or inspection lapse if the car isn't being driven.

True in many states, but California wants vehicles to have valid registration even if the vehicle is not at all operational. The owner gets a break on the cost, but Sacramento still wants their due.

https://www.dmv.ca.gov/portal/vehicle-registration/vehicle-r...

[Ekaros]

Nothing prevents DMV and police having interconnected database. Which would allow automatically communicating vehicles removed from use or with lapsed MOT.

[jcrawfordor]

They do, the stickers are mostly to make it easier for cops to see expired reg without having to type in every license plate or have a vehicle fitted with LPR. They can also look up the vehicle status in the DMV database, and via an interstate compact.

[reaperducer]

That works fine if you live in a tiny country. But in the United States there are tens of thousands of law enforcement agencies authorized to write tickets. Some of them are massive organizations like state highway patrols. Others are towns of 200 people, or even individual schools that don't have the time, money, or infrastructure to integrate with a national system.

[WkndTriathlete]

The tag is to indicate that the vehicle has paid the appropriate taxes for using the roads. If the tax on the vehicle is not paid then it should not be used on the roads.

Without the tag there is no way to enforce that without the police having to manually enter the plate number for every vehicle they see. Hence the tag: if the police see a vehicle without an up-to-date tag applied it is not legally allowed to use the roads since the owner hasn't paid to keep the roads maintained from the wear incurred by the vehicle while driving on them.

There is an argument to be made that the police could simply use a system that reads license plates up and checks the information automatically, but there are so many 4th amendment abuses/workarounds that the police already use it's hard to imagine much public support for such a system.

[fps]

The US's anti-surveillance laws and sentiment keep ubiquitous camera systems from existing in many places, and keep the ones that do exist, quiet. In my state, Massachusetts, traffic cameras legally cannot be used to issue citations. Automated toll collection, which uses highway mounted plate scanners, faced substantial backlash from people for privacy reasons. And Massachusetts is one of the least anti-government states in the country. If it got out that the police were monitoring which cars were on the road and how often they were driven, there would be literal riots.

[ezfe]

In Massachusetts they do, but we still use those stickers as another form of labeling.