I am on the service road by Richland Mall in Waco. There are three lanes. I am in the middle lane going straight. There is an exit off Highway 6 (Sanger Exit) onto the service road. Several times vehicles coming off Hwy 6 come off and go across all three lanes to enter the mall. There are no yield sign posted on the service road. Who has the right of way when there are no traffic signs.

The general rule of thumb is that through traffic has the right of way. If it is not safe to cross lanes of opposing traffic then the driver is operating a vehicle in an unsafe manner (wreckless driving) and can be sited as such.

I stumbled into this law (called a rule here but I saw it in MD Transportation Code) when I worked for the Highway Division in MD. I know nothing else about it but wik says many states have similar laws but not necessarily called the same.
Boulevard rule - Wikipedia, the free encyclopedia