Visual Aids for Cycle Training

This may not be what you mean by 'maintain your speed' (you may mean maintaining the your own speed rather than that of other traffic), but I wouldn't say that you have to maintain the speed of other traffic. You should make your risk assessment based on the speed differential between faster traffic and you (just as you should make it if you are the faster traffic). Speed differentials are fine as long as they're not too extreme, e.g. if everyone around you is doing 70mph, you'd better adjust your risk assessment. :)

Indeed - you don't have to maintain the speed of other traffic (it's often wise to back off), but you do have to operate within the contraints of your cardio-vascular system!