On the interstate, you rarely come to a complete stop. You do often have to slow down substantially waiting for cars to get over to let you in. It does take a while to get up to cruising speed from that speed. Basically, you're at the mercy of other drivers, during heavy traffic times, which is much of the day. People will usually get over, when they can, but that isn't something that you can assume. Some people simply refuse to leave the right lane, when their exit is coming up soon. You can't just keep on rolling & expect people to get over, unless you're okay with getting in an accident. It's on the car entering the interstate or highway to get on safely. Those that are driving at full speed are looking out for themselves.
Getting off the interstate, where there are yield signs, is where you're likely to have to come to a complete stop & wait for a break. These roads have traffic lights periodically, which open up space to enter eventually. There are cars entering from other locations that are also taking advantage of the traffic light, so you may have to wait through more than one light, but you'll eventually get a break in traffic.
I must not be explaining the situation well or something. It surprises me that so many of you seem to doubt that you may have to slow down or stop to merge into traffic. I realize the traffic here is worse than most locations, but there has to be others here who deal with a crazy amount of traffic on a daily basis.