My answer to them would be this: such a tangent is irrelevant to the discussion at hand. I merely pointed out the failures of such proposed reduction in speed laws. It's up to you to tell me how you are going to get the reckless drunken drivers to obey the posted speed limits. For that matter, it's up to you to tell me how such speed limits are going to prevent drunken drivers from driving in their intoxicated state in the first place...
Instead of doing that, why not go after the drunken drivers, put them in jail, take away their driver's licenses, etc? Wouldn't that be more productive than punishing the law-abiding? I can guarantee you that my method of dealing with drunken drivers is going to be far more effective at curbing drunken-driving accidents on the highway, than any number of speed limit implementations.
The same holds true for weaponry. Law abiding people aren't going to use their weapons, or whatever accessories are used with such weapons (such as flash suppressors that prevent the marksman from being blinded by muzzle flash, an adjustable shoulder stock that allows people to adjust their rifle position in an optimal manner for their bodies, etc., or any of the other "evil" features that were part of the 1994 Assault Weapons Ban).
Limiting magazine capacity for law-abiding people does absolutely nothing to stop criminals from committing crimes, since you're attacking the wrong crowd.
Even if you did ban law-abiding people from owning 11+ round magazines, there are millions of 11+ round magazines still out there. How are you going to get the criminals to stop using them and turn them in? How are you going to prevent them from buying more from illegal arms dealers?
Even if you were able to magically make criminals turn in their 11+ round magazines, what about the ability to carry multiple weapons, like Robert Deniro's character in Taxi Cab Driver?
Again, if the anti-gun crowd can show me a way how such legislation would make criminals obey the laws, then I'd give their assertions much more weight. Until then, maybe they should start turning their attention to criminals, instead of the law-abiding?
The arguments of such individuals sometimes reminds me of a former Ohio Senator, Howard Metzenbaum:
Dave Kopel also wrote a very in-depth article, pointing out the fallacies of the above arguments:
http://www.davekopel.com/2A/Mags/Banning-Assault-Rifles-Won't-Work.htm