TSA is changing rules in April to allow small pocket knives and some sports equipment as allowable carry-on items. http://www.cnn.com/2013/03/05/travel/tsa-carry-on-changes/index.html
The article has some nice graphics from TSA to cover what is and isn't legal; the graphic for knives even includes a dual unit ruler. But dimensions reflect an odd mix of units. Pocket knifes must have a folding blade less than 6 cm (2.36") long and less than ½" wide. That units mix could ONLY make sense to a US bureaucrat. Many full-size sports equipment items are allowed, but baseball bats must be less than 24" and 24 oz (no metric equivalent offered), therefore toy bats. I guess the real question is why did they choose metric for only one dimension. Does anyone know the international rules they claim to be "aligning" with? How are visitors not familiar with US Customary supposed to figure out what they are talking about? Given that international visitors are also subject to these rules, shouldn't TSA ALWAYS use SI units. To me, the only debate is whether they need parenthetical Customary units in addition. (I vote no because SI is "preferred," but I'm flexible.)
