I've never had it on a car rack, but my SMGT from chainring guard to rear mudguard is 1.9m; about 10cm longer than my Dawes Discovery (which is not a big frame) from front tyre to rear mudguard. It usually fits on trains, unless the cycle space is one that isn't designed to accommodate an adult's road bike with the steering straight. USS bars are certainly a pain width-wise (they're no wider than a MTB bar, but in an awkward place). When advanced tetrising is required it's possible to overlap by hooking the top-tube of a suitably shaped DF bike over the USS handlebar. You could probably remove the seat to gain a fair bit more wiggle-room (and a dry seat).
Weight is about 24kg, so while on a good day I can hang it by the rear wheel in a dangly bike space, a roof-mounted rack is right out. I wouldn't expect much drama with a towbar-mounted rack.
Other recumbent bikes can be a lot longer than a Streetmachine. I think the Red Baron is of the order of 2.1m. This post refers:
https://yacf.co.uk/forum/index.php?topic=79449.msg1630898#msg1630898And obviously taller riders may have some scope for reducing the overall length by pushing the boom in further. Would recommend some sort of alignment marks to make it easy to reproduce the right extension/rotation.
(In general, other than for BHPC events, car travel tends to cause me to resort to something more convenient to fit inside like the ICE trike or Brompton.)