Broadly, I agree. But there are two very distinct groups of laws that you're groping together here:

Firstly, there's federal law, which is primarily responsible for what we think a modern safe vehicle is: crash testing, airbags, backup cameras, tire pressure monitoring, antilock brakes, stability control, etc. These laws primarily apply only to new vehicle manufacturers, enter enforced against those manufactured by the federal government.

Second, there's state law. Basically, all traffic laws are under state purview, and enforcement. The safety equipment required under state law is generally extremely basic. In most states you can qualify with as little as: DOT rated tires with tread, at least two mirrors, turn signals, seat belt, headlights, tail lights, horn, a front windshield, and a functioning wiper. These are the laws that traffic police enforce.

> Let’s also assume that some of those safety standards are not immediately visible from the outside - eg, it’s difficult to tell at a glance if the frame of the banana car is a well-constructed piece of welded steel or a shopping cart.

Because state vehicle safety law is generally very basic, it usually is possible to tell from the outside whether equipment requirements are met.

Also, shopping carts are made out of welded steel. And besides, it is entirely legal to use wood in the construction of a vehicle. State law typically does not prescribe the types of materials used beyond some extreme generalities in their performance (e.g. visibility through windows, structures physically attached as opposed to being loose). Generally, state law only cares about operational safety, they don't really regulate design safety. Horrible unsafe designs that would fail a crash test are only federally illegal for manufacturers to make and sell to people.

But let's entertain your scenario for a second. Let's say that there is something about a vehicle that fails safety standards that isn't visible... legally you there's no way to pull the vehicle over for a reason that you don't know of... because you have to know of a reason to justify the stop to begin with -- you'd have to find some other reason.

Ideally, the way you'd enforces vehicle safety for these kinds of scenarios, and the way that the rest of the world handles it -- is to require vehicles to be inspected. But only 15 states have chosen to require periodic passenger vehicle inspections.

> A cop sees what is clearly a hand-made banana car driving past them in the other direction on the road. What do you feel like are the appropriate actions for that cop to take in those circumstances, with just the facts available to them at hand?

They should do everything that their state law enables them to do, including:

* visually confirming the existence of required equipment: lamps, windscreen, signals, road tires, etc.

* confirming the display of any credentials required by their state: inspections stickers (if applicable), registration stickers (if applicable), license plate (believe it or not... if applicable), etc.

... and if they don't meet these requirements, or they break other rules of traffic operation, initiate a stop and investigate further.