Diesel Particulate Filters.
 

It would appear that many UK motorists are driving about without Diesel Particulate Filters. The current MOT doesn't always pick up if it has been removed, and though illegal to drive a car without one it is not illegal for a garage to remove it. A change to the MOT will improve the situation, but there are also calls for spot checks to be carried out on diesel vehicles (as there currently are for checks on the fuel being used).

'Thousands' driving without crucial diesel filters - BBC News

DPF filters decrease fuel economy.
And they still create DPM in the most dangerous particle size of 1 to 2 microns.

I say if you are not in a city with a smog problem lop them off.

That's one of the many differences between the wild west and the UK.

I'm glad it is.
If paying more for fuel and getting worse fuel economy helps you sleep better at night go for it.
But you are producing way more CO2 with the DPF.

I have no choice what I pay for fuel. Nor am I advocating DPFs. I am just saying we can hardly move in the UK without passing through a large town or city. I live 30 miles from Scotland's capital, Edinburgh, with some real pollution hotspots in the city. Edinburgh is a smaller city than Albuquerque. The law here does not allow the removal of DPFs and I believe in the rule of law, however inappropriate that particular law is. I am just happy I don't drive a diesel, and never will.

They'd never get away with spot checks in the US. Can't pull someone over with no evidence of a law being violated.

This is the UK law on Spot Checks:

The police can stop a vehicle for any reason. If they ask you to stop, you should always pull over when it’s safe to do so.You’re breaking the law if you don’t.
If you’re stopped, the police can ask to see your:
driving licence
insurance certificate
MOT certificate
If you don’t have these documents with you, you have 7 days to take them to a police station. You’re breaking the law if you don’t show the requested documents within 7 days.
The police can also give you an on-the-spot fixed penalty notice for many minor offences and make you take a breath test in certain circumstances.
You can also have your vehicle seized if you’re stopped on suspicion of driving without insurance and for some other offences.

Usually they stop you for a lighting offence or less than ideal driving.

You can be pulled over for emissions spot checks if you are a commercial vehicle.
 

Here in California, people can report you via a State hotline if you are deemed too smokey, even if you are not a commercial carrier. As many of you know, we have several large population centers nestled in valleys that trap the tailpipe refuse and bake it in the California summer sun. Emission controls have been a tremendous benefit. However, though I agree with Oil Pan 4's position that many areas do not need our draconian measures as implemented here in California, the emissions technology of diesels is moving forward and is really in only it's second decade of development. There will come a time when diesels will be as clean and reliable as their gasoline counterparts. Of course, in the environs of the Southern California Valleys, air quality is sensitive to the smallest of inputs and thus, the draconian measures. This does make California a test tube of sorts for the rest of the world as the populace and government seem willing to pay the price of these measures.

Your statement is applicable to current technology.
 

There will come a time when DPFs will be of no more hindrance to performance as 3 way catalysts are to gasoline spark ignited engines.

And DPFs don't necessarily "create" small micron particles. The whole system of DPFs and high pressure fuel systems in diesels biases particulates toward the sub micron range. I think you are aware that gasoline direct injected engines also create these single digit micron particulates. They are under scrutiny and may need remediation as GDI is becoming ubiquitous.

I thinks time will come when this is the case, but I believe that cars will steer well away from diesels before then. Even now, here in Europe, the appetite for diesels is drying up with secondhand prices dropping.