AT the US 377/SH 183 roundabout in Fort Worth, Texas, at one entrance to the roundabout, traffic is allowed to enter without yielding, and traffic in the rounabout has the yield signs. I assume that this was done due to such a high volume of traffic entering the roundabout, that standard rules wouldn't work well and would cause a backup.
I hate this setup and do not agree with it at all. When you are in the roundabout, it is very unpleasent to turn your neck that far around and hold it here looking for a break in traffic. I can't imagine the driver of a large truck being able to see in that direction due to the design of the truck.
Not only that, as you approach the roundabout there is no guide or warning sings indicating the roundabout. The only guide signage is route marker assemblies at the roundabout exits.
http://maps.google.com/maps?q=fort+worth&hl=en&ll=32.718437,-97.44283&spn=0.00658,0.009645&hnear=Fort+Worth,+Tarrant,+Texas&t=m&z=17&vpsrc=6&layer=c&cbll=32.718421,-97.442705&panoid=p3CMRnW3mbK__R14RzGhzQ&cbp=12,295.01,,0,0 (http://maps.google.com/maps?q=fort+worth&hl=en&ll=32.718437,-97.44283&spn=0.00658,0.009645&hnear=Fort+Worth,+Tarrant,+Texas&t=m&z=17&vpsrc=6&layer=c&cbll=32.718421,-97.442705&panoid=p3CMRnW3mbK__R14RzGhzQ&cbp=12,295.01,,0,0)
That intersection does not appear to be a true roundabout, but more of a traffic circle or rotary--I say this because of the long merging/departure areas that appear to exist.
Since it's not a roundabout, the typical yield rules of a roundabout can be thrown out the window...
I can think of at least one traffic circle (it's not a roundabout) in Washington, DC, that requires traffic already on the circle to yield. It's located at the western end of Arlington Memorial Bridge. Certain movements get blocked off at certain times of day to prevent tailbacks from locking the thing up. The vast majority of the traffic (probably 95% or more) merely touches the circle at the "corners" (for lack of a better term).
A similar situation happens at the Somerville Circle in NJ (US 206, NJ 28, US 202). Traffic in the circle actually has a STOP sign, with through traffic on US 206 getting priority. (NB US 206 has to yield to traffic in the circle.) I was in the area for a summer, and the circle and nearby junctions with US 22 and I-287 were a mess, especially during rush hour.
A similar situation happens at many circles in NJ. Flemington Circle (202/31) is another that leaps to mind. NJ has been getting rid of a lot of circles lately, at least.