Thank goodness you are going to be OK. My wife and kids were rear ended a few years ago while at a stop and were pushed at about 35mph in to the car ahead of them. The airbags didn't go off then either and we thought something was wrong also. It turns out the sensors are very specificly located to comply with the point of impact. Since you were hit by a roadster the point of impact would probably have been so low the airbag wouldn't have helped, and possibly might have harmed you when it went off. Hope you get to feeling better soon.