Every time I drive north on MoPac I pass this unmarked exit just south of US183. The exit only puts traffic on the frontage road, but the frontage road doesn’t intersect any other road until it passes US183 and gets to Loop 360. Both US183 and Loop 360 have dedicated marked exits so I’m not sure what this exit is for. The exit lane from MoPac becomes the left-most lane on the frontage road, which then becomes an on-ramp for US183 (for northbound traffic coming from Steck Avenue/Anderson Lane/Spicewood Springs Drive). Once on the on-ramp and flyover, the two left lanes are for US183 northbound and a single right lane (the on-ramp lane) becomes the US183 southbound exit lane.
I believe it would be in good intentions if this unmarked exit was marked as the US183 southbound exit from MoPac since it pretty much puts traffic into the US183 southbound exit lane on the flyover anyways. That way there isn’t a giant kerfuffle at the actual US183 exit (people just don’t understand how to maintain their speeds at that point).