The Maryland Boulevard Rule holds that motorists on a through highway have the right of way over vehicles entering the road from side streets, driveways, and similar locations. The Court summed it up pretty succinctly: "The purpose of the Boulevard Rule is to 'facilitate the free flow of traffic on major thoroughfares by preventing interruptions or delays and insuring the safety of the drivers there."
This means that vehicles entering the roadway, whether from a side street, driveway, dirt road, shoulder, or other similar location, must yield to vehicles already on the roadway. It is not uncommon, however, for a motorist who enters a main road from a parking lot, for example, to claim that the car on the main road "came out of nowhere" or was speeding. Under the Boulevard Rule, the Court will not consider speed as long as the vehicle already on the roadway is there lawfully. As the Court of Appeals describes it, they will generally not consider "nice calculations of speed, time or distance lest the purpose of the boulevard rule, to accelerate the flow of traffic over the through highway at the permitted speed, be thwarted."
In short, the vehicle on the main road has the right of way over vehicles entering the main road. It does not matter how fast that vehicle is travelling, unless the speed is extremely excessive.