I really don't believe in the two second rule, even in staggered formation. There are too many incidents of bikes in this tight of a group hitting each other in case of a problem, and there is no room for the inevitable intrusion...even with a small group. I have been converted to the 3-5 second interval...in good weather. 15 seconds is way too much, however. That is over 1,500 feet at 70 mph! If you leave more than a quarter mile between bikes, you will lose the group, have endless intrusions, and lose the visibility advantages that group riding affords. You may as well be riding alone. Leave enough space to avoid trouble and stop safely, but don't stretch it out so far that you become visually distinct from the group. Just my humble opinion.