The government can then know you use Signal. This may be problematic in heavily autocratic regimes, but besides those, what threat scenario are you arguing for here? The Sealed Sender concept disallows building a social graph. However, you can utilize a VPN to mask your point of origin or, if necessary, even use a burner number. Under the worst case scenario that the US gov takes over the whole AWS infrastructure and tries to correlate connections to users, there’s still very high information entropy. At that point, we’re talking about the US gov as a targeting threat actor. If that’s your opponent, you shouldn’t use everyday customer electronics or applications anyway. That’s some spy shit, even domestic activists won’t fall under that much scrutiny.
Doing 50 that close to other cars would also be very uncomfortable.