First, the roads are connected in sequence:
Personally, your question is divided into two aspects. On the one hand, it is a parallel connection with different widths on the plane, on the other hand, it is a parallel connection of two roads with different elevations.
The so-called smooth connection means to have a transition and try to avoid traces of rigid connection. For example, the plane should change gradually from wide to narrow, and the interface should not have obvious angles. The elevation of the pavement should also change gradually, and there should be no hard bumps at the interface, so that people will not feel the elevation change when driving (walking).
Second, the road bed, road bed:
Subgrade is the foundation of road.
In the range of 300 mm below the road surface, it is called a road bed; The range of 300 mm to 800 mm is called cross-country bed.
Iii. About dikes and embankments:
The fill above the natural ground is called embankment.
The fill within the range of 800mm~ 1500mm below the pavement is called embankment; The lower part of the upper dike is called the lower dike.
Four, subgrade high-rise:
To tell the truth, I have never seen this word, and I don't know its meaning. Is it a mistake of "subgrade elevation"? Or am I really a layman, rare and strange? I hope the netizens in the industry will correct me, and I can also increase my knowledge with you.
Have a nice day.
Supplementary answer:
Subgrade elevation refers to the design elevation (also called elevation) of the top or bottom surface of a foundation (or base) of a road.
If you need to know the concept of facade, please search related terms in Baidu Encyclopedia. Due to space reasons, I won't go into details here. Please forgive me.