Here's some clarification on the D Barrier at Download, thanks to Dan and John for the info.

"Due to the fact that the new arena  is on a slight slope we have been advised to install a secondary barrier, or 'D Barrier' to reduce the risk of forward movement and possible crushing on the primary barrier.

However, the ends of the barrier will be open (as in wide open as it is designed only to reduce forward movement) with no restriction as to who can go between the primary and secondary barrier."