Asklemmy

Or be neighborly and go over with a couple big blocks and tell them you noticed that's sketchy and wanted to help out!

I know OP said they would go talk to their neighbor but I just wanted to add an anecdote. I worked at a place that had a bunch of trailers, and it wasn't uncommon for someone to need a vehicle and just leave the trailer in the parking lot. One time someone didn't secure it and it started to roll. A new guy who was trying to be the hero went to stop it and got his thumb crushed between the trailer and a steel beam of a pavilion. He ended up loosing his thumb. So yeah, chock your trailers and don't try to catch a run away one.

Not only does it need blocking on the wheels, but that thing's also apt to be stolen with the hitch just sitting there like that. They might consider parking a car in front of it too.

Absolutely. It's a public safety issue. Just don't come off as rude and they should appreciate it too

My mom got run into by a trailer that was rolling down a hill. Luckily my mom decided she was a superhero that day and was able to push the trailer to the curb to stop it from rolling down any further (it was a pretty steep hill). She got away with only a broken ankle.

So I say yeah. Tell them.

Park an old car across the street for an insurance claim

It looks like it gives birth to Cybertrucks.

Chock the wheels and leave a friendly note on the hitch if nobody is home IMO, I seriously doubt they would take offense.

Cybertruck

Go for it, use a personal anecdote if you have one about how you or someone you know was bit by not putting blocks in front of the wheels.

If you don't have one... make up a white lie about how your now deceased great uncle Johnny told you he left a trailer out without blocks on a little slope like that and it rolled into the street.

The anecdotes / a story can be a ... easier path to selling the truth that can feel a bit more humbled vs "I know better than you, so do what I say."

If you were my neighbour I would love it if you notified my of the danger. I'd also say that if they give you a negative response they will move soon so there's no risk really

Maybe something like "Hey neighbor, I noticed your trailer isn't fully secured, I've got some wedges to block the tires if you don't have anything"

Trailer emergency brake is probably pulled. Not ideal, but it’s probably not gonna go anywhere even if they start to load it like that.

Why wouldn't you? I suppose they would be thankful because you show considerations about their stuff and wellbeing. Some wooden blocks in front of the weels should probably be enough.

Never hurts to be a good neighbor, and u don't want it rolling when ur trying drive down the street.

If I saw them I'd say something. But I wouldn't go out of my way. You have to learn the hard way sometimes. But I would definitely point my security camera over just incase.

Two questions: do you like these people, and is anything anywhere near where it could go if it starts rolling?