Am I being too overprotective?

Well at 23 you can’t really stop her.

Seems like quite a reasonable plan to me, and she's kind to include her sibling. I imagine they'll have a great time.

She might find she can’t rent a car if she’s only had a license for 6 months and she’s under 25. Or if she can, it might be prohibitively expensive.

Meeting a random man she met online and has never met face to face?! Face time doesn't count imo

I'd be the same as you op... I'd hate this idea! Could you make the road trip into a family trip and all do it?

If not, I'd seriously try and sway her away from having the internet bloke meet them and just make it her and her brother

Under 25 insurance is usually very expensive.
That said if she’s 23 and can afford it I don’t think you can stop her. Just encourage sensible precautions.
Has she ever been to USA? One look at size of some freeways and the traffic and she may well change her mind.

She is meeting a random man out there, too?

I would not be happy.

Why are you 'trying' to give her independence? Can't she just 'have' independence?

Obviously we don't know your daughter, you've said she is neuro diverse, how much is that limiting her ability to be the adult she legally is?

The roads are not like here. Last time we drove Route 66 it was empty.

Yes and the cars are automatic. She's right driving there is easy, except in very busy areas like those on the approach to an airport.

You can give her advice but you can't stop her at the end of the day. And actually the driving will be easy, those roads are very open and not busy at all, she'll basically just be driving straight for miles on end with hardly any other traffic.

I on the other hand had motorbiked round Vietnam around that age and my parents didn't even know!

How ND is she, for want of a better phrase! Is she independent at home and does her own thing?

I think the trip with her brother will be fine, sounds fab. Not sure about the older fella though! How much older and how well does she know him.

She's an adult. Not even 'only just' an adult - she's a 23-year-old woman. You can't stop her from driving wherever the hell she wants or seeing whoever she wants.

I would also let her 17-year-old sister go with her if it was just the two of them. But certainly not if the trip is going to include a meet-up with a man your 23-year-old has only ever chatted to online.

Yes, the first time I ever drove an automatic was doing the Route 66. I remember being initially nervous picking up the car from Avis on a busy road in San Francisco, but after that it was fine.

Driving in LA is horrible in my experience. I was happy doing some of the more long haul driving as, as others have said, there can be great stretches of wide, quiet roads if you’re going some distance. In the built up parts of LA, my American friend had to take over as she was used to the congestion and would not be bullied in traffic. Parking in both LA and San Fran was also bad in the more congested areas and my American friend had to verbally fight it out with people who had eyes on the same parking spec for example. I’m just not cut out for it and did not want to be getting into fights as a foreigner. Advise her to consider the route carefully in terms of how busy areas are likely to be and how to manage bullies in both traffic and car parks.

I drove from Las Vegas to the Grand Canyon a few years ago, not entirely along Route 66, but doing some of it.

I agree with others that the driving was a doddle, and the road was practically empty. Driving in Las Vegas was also easy (even though the roads were not empty).

Is she aware of how to drive there?

Stop signs that you HAVE to stop at, minimum speeds in some states, 4 way stops and how they work, turning right on a red, can't change lanes when on a junction and other differences?

She might want to look into the cost of hiring a car as she is under 25, it is generally quite a bit more with possibly a daily surcharge. Also the insurance is massive in the states. If she does drive an automatic, tell her to remember that her left leg doesn't do anything - I found it easier to make the switch once I realised that.

As others have pointed out, she should maybe gen up on different etiquette at road junctions etc/Highway Code etc.

You’ve posted about this before? Or someone has posted a similar thread?

Mate, you need to cut the purse strings. She has a job, is an adult. Sure she finds things hard sometimes, but she needs to make her own decisions and deal with the consequences. Stop being so involved

I have been driving 30years and felt overwhelmed (which I'm not easily) when driving an interestate in Atlanta that was 8 lanes wide on each side.

If they are doing the old Route 66 tell them to check out Uranus in Missouri 🤣

Went to the US a few weeks back, first time we've ever driven abroad (I say 'We', DH did all the driving). We have both been driving years but still found it daunting and I don't think I'd want to do it (I'm not keen on driving in the UK though!). That being said, I felt it was probably easier to drive over there. Lots of long, straight roads that were only busy around cities. Road signs etc are pretty logical - but worth reading on on first (for example you can turn right on red if it's clear at most junctions) as they differ to ours. Fair play to her that she wants to do it. (I would be extremely worried too though OP!)