This sign has since been replaced with something more 'active'; have a look at
http://www.duke.edu/~mjt7/fbroad/durmass16.jpg
The sign now reads \95/TUNNEL PROHIBITIONS HAZMATS EXIT NOW.

It is possible that the wording was changed as a result of hazmats not seeing the old signs, or the sign may have been changed due to someone determining that there was not enough warning, even if no cases had actually been observed of hazmats entering the tunnel.

A quick call to the Maryland State Highway Authority might answer this question, or it may muddle things up further.