Happens to me all the time even when merging onto the hiwy.

Then they flip me off as if I were wrong for trying to merge.
I will speak to IN Code, but I believe the same/similar principles are found in all state laws.
To be specific, it is the responsibility of the merging traffic to do so safely. Despite the typical attitude of a driver than they have a "right" to merge, that is not correct. The existing traffic is not required to yield the right of way to merging traffic; rather, it is the responsibility of the merging traffic to find a safe opportunity to do so.
The responsibility to yield is found in IC 9-21-8-31(a)(2):
A person who drives a vehicle shall do the following: Yield the right-of-way to other vehicles that have entered the intersection from the through highway or that are approaching so closely on the through highway as to constitute an immediate hazard.
The only exceptions to these would be if there was either a LEO or other authorized worker manually directing traffic, or specific signage otherwise contradicting the established code.
Further, when someone desires to merge into another lane, they must do so such that they do not create a condition where the traffic in the intended lane is now in jeopardy of unsafe operation (reduced stopping distances or unsafe following distances). In short, you cannot legally merge into the other lane by cutting someone off. I see this very often ...
Example: traffic is flowing at 70mph in a 70mph zone. Vehicle "A" is a smaller car and traveling in the left lane of a multi-lane highway; Veh "A" is safely following another vehicle (Veh B) by a margin adequate to satisfy the State following distance requirement. (often described as one vehicle length per 10mph of travel speed). So Vehicle A is perhaps 100 feet behind vehicle B. Now, vehicle "C" now wants to merge into that lane, and cuts off vehicle "A" simply because they can fit the car into the same lane. But what they have done is broken the law by unsafely causing the following distance of vehicle "A" to be unreasonably reduced. People often reason that because they can "fit" into the space, it's their right to do so. But that is patently wrong, and typically illegal. You must "merge" by respecting the full operational distances of following others. Just because you can cram your 20' long truck into the 100' of safe following distance, does not mean you have a "right" to do so. And in fact, it is illegal to do so in any state I know of. By inserting your 20' vehicle into the minimum 100' required distance, you have essentially created two insufficient distances (you following too close behind vehicle C, and you reducing vehicle A behind you to a new unsafe following distance).
In Indiana, the following distance requirement is defined in 9-21-8-14(b):
A person who drives a motor vehicle may not follow another vehicle more closely than is reasonable and prudent, having due regard for the speed of both vehicles, the time interval between vehicles, and the condition of the highway.
When you enter into the minimum safe following distance requirement of another vehicle, it is you who is at fault by violating the lane of traffic you merge into.
I realize this happens, well, pretty much all the time. Many folks think that because they turn on their blinker/signal they have a "right" to merge. But that simply is not true. The merging traffic is required to do so safely; the onus is upon them. Generally, the desired lane of traffic has no responsibility to yield to the merging traffic.