1. Historical reasons: Inner Ring Road was formed in the early stage of urban planning. At that time, the forecast of traffic demand may not be accurate enough, or the technical and economic conditions of road construction are limited, resulting in the width of the inner ring road unable to meet the subsequent traffic demand.

2. Urban planning: The construction of Inner Ring Road is closely related to urban planning. In the process of urban planning, the determination of the width of the inner ring road and the location of the line may be restricted by many factors such as topography, architecture, transportation hub and so on.

3. Land resources: For some cities with tight land resources, such as Shanghai, the construction of the Inner Ring Road may be limited by land resources. With limited land resources, it may be difficult to expand the width of the inner ring road.

4. Environmental protection: In some cities, especially those with a long history, the construction of the inner ring road may be restricted by environmental protection. In order to protect historical buildings and landscapes, urban planners may limit the width of the inner ring road.

In a word, there are many reasons for the narrow inner ring in Shanghai, including the comprehensive influence of history, urban planning, land resources and environmental protection.