That's what on-ramps are for. If traffic is congested enough to demand merging (rather than just making sure the road is clear), you shouldn't be driving at 90m

That's what on-ramps are for. If traffic is congested enough to demand merging (rather than just making sure the road is clear), you shouldn't be driving at 90mph. On the other hand, you probably shouldn't be driving at 60 either. Speed limits should be limits, not averages, or worst-case.