There are yield signs. That can sometimes mean stopping if necessary. It's unsafe, but sometimes trying to merge into traffic when no one will let you in is also unsafe.

There are a lot of different types of on ramp situations so there is probably no one-size-fits-all solution that will always be safe.

There's an on ramp in Vermont that has very low visibility to see what sort of traffic you need to merge into until you get past some rocks. On the highway you're trying to merge into there are signs for through traffic to stay to the left, which helps a little. But a lot of people need to stay in the right lane because they're planning to get off at the next exit a quarter mile away. For some reason the people using this on ramp seem to think the solution is to come flying out of the on ramp at about 80+ mph (speed limit on that section of highway is 55 mph).

There are lots of unique situations out there.