2006 prius comes to a stop for no reason

welcome!

any dash lights? you can check the inverter pump, but i'm not sure that would make it go into park

 

Any trouble codes to share. If any of the trouble lights on the dash are illuminated, there are codes to read.

 

If no check engine type codes no lights on the dash or mfd that means it’s strictly mechanical.
I would investigate the manual parking pawl in the trans.

That’s super dangerous. And hard to believe it doesn’t damage the trans doing that at 50 miles an hour. Hope you didn’t spend much it may need new trans.

 

I suggest that you have it towed to a dealership and pay for a diagnosis of the problem.
Once the cause is known, THEN you can talk about service bulletins, etc.
And you also can talk about reporting the condition to the NTSB/NHTSA,

But given it's age and mileage, unless there already is a "program" in place for that particular failure,
the repair cost likely will be 100% on YOU.

 

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Check if the inverter coolant pump is functioning by touching it with a pencil/rod/finger. Next see if there is liquid movement in the inverter coolant reservoir attached to the firewall. If neither, then check the AM2 fuse under the hood to see if it has blown.

If the pump or AM2 has failed (no vibration or liquid movement), replace the pump first, then the AM2 fuse, otherwise it will blow again.

 

A failing 12 volt battery or a failing 12 volt connection can cause this as the voltage drops off it will go into park by itself. The Park Pawl is held open by the car. When you remove power or if the power is fluctuating it can engage the park Pawl.
also a failing 12 volt battery can cause all kinds of weird things to occur.


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Given in the OPs case the car was running, to arrive at this, the inverter DC/DC converter would also have to have failed, but, even before the 12-Volt battery could have reached low enough voltage to allow the parking pawl to engage, there would be a dash full of error lights.

Somehow, I can't picture this as a viable explanation to explain the OPs situation.

This sounds like a fail-safe reaction to something very serious.

 

Not while it is in "ready" mode it shouldn't.
The inverter should still supply power for the 12 V systems.