In general, do cars usually understand or notice arm turn signaling?

Is there any good signalling options for when your hands are occupied (biking uphill)?

There's one four way stop I'm always unsure what to do because it lacks part of the sidewalk ramp, so I basically I need to get into the middle of the intersection to turn left (then essentially going straight), which is also partially uphill so I don't usually feel comfortable letting go of the handle bars to do hand gestures.

Plus I'm 90% sure most of the cars don't even realize its an on-ramp to a MUP so I don't think they are expecting 1) a cyclist or 2) a cyclist going into what they think is a dead end (looks like a T to drivers).

...it's a really awkward intersection.