With the advancement of China's reform and opening up to the outside world, the number of vehicles in cities all over the country has risen by a substantial margin, and inevitably, the traffic flow increased dramatically. In consequence, traffic congestion and low driving speed have become very serious problems, intensified by the interference between motor and non-motorized vehicles and between vehicles and pedestrians. Hence, an urgent demand for grade-separation structures in urban areas as well as on highways has been brought up so as to divert the traffic, to raise traffic capacity and to ensure pedestrians' safety.

The early grade-separation structures in China are mostly subway crossings under railway tracks. However, in recent years, thanks to the technological progress, China has been able to build under-crossings with the application of the advanced jacking method of launching prefabricated box, which can obviate the necessity of interrupting the road traffic. In cities like Beijing and Tianjin, a great number of technically complicated under-crossings of large-sized box structure have been successfully built.

China's first grade-separation structures came into existence with the erection of urban road bridges. In 1959 when the No.1 Jianghan Bridge was built in Wuhan, allowing the riverside road to pass through its side span, the first half-cloverleaf grade separation was completed in our country. In 1965, over-crossings in the same style were built in three different places where the main roads meet the riverside road along the irrigation canal between the Miyun Reservoir and Beijing. 1964 witnessed our first rotary interchange -- the Dabei over-crossing -- erected in Guangzhou on the basis of the construction experience and technique of highway and railway over-crossings.

In 1980s, with the growth of urban construction and development of highway transport and communication, numerous beltways were built, which can provide great traffic capacity and allow high vehicle speed. For instance, Beijing's Second and Third Ring Roads and East City Expressway, the intermediate and Outer Ring Road in Tianjin, and Guangzhou's Inner and Outer-ring Road and viaduct. In Shanghai, the elevated Inner-ring Road is now under construction in Puxi while the outer beltway is under project. Subsequently, there has appeared an upsurge of erecting different-sized grade separation structures on urban main streets and express highways. Up till now, in Beijing alone, 80 odd large over-crossings have been erected, which makes the city rank the first in the whole country both in number and scale.

Overcrossings differ in form and style: cloverleaf, diamond, trumpet and rotary type as well as interchanges, directional interchanges and complex interchanges. Most of the structures consist of two or three levels and some even of four levels. Complicated in style and structure, overcrossings usually include the structures of curved bridge, inclined bridge, skew bridge and special-shaped deck, etc. For example, the new technique of pre-stressed concrete curved bridge has been employed in the erection of the Tiegang Overcrossing in Qingdao (completed in 1986) and the curved box-girder interchange of the east approach of Nanpu Bridge in Shanghai (completed in 1991).

Meanwhile, pedestrian overpasses have mushroomed at bustling crossings in cities. They are efficient means to ease traffic congestion, raise the traffic capacity for motor vehicles and guarantee pedestrians' safety. With narrow spans and widths and small load-bearing capacity, pedestrian overpasses can be of various structures, simple, lively and flowing in form and in harmony with the surroundings. They can not only satisfy the traffic demand but add enjoyable scenic views to the city as well.