| Under the macro-policy background of new-type urbanization,national governance system modernization and 'Five-in-One' general planning,etc.,and facing increasingly severe urban traffic congestion,the idea of transit city construction is proposed in many cities and regions.With the planning paradigm changing from 'incremental planning'to 'inventory/reduction planning',the urban-rural planning methods have undergone changes from traditional 'goal orientation' to'problem orientation',how to solve urban traffic congestion in urban-rural planning and urban traffic planning by means of spatial planning design has become a problem confronting workers in urban and rural planning.The urban-rural planning scholars at home and abroad have not stopped the discussion on how to solve the problem of traffic congestion,and the more widely recognized and consensus view is to greatly develop large-volume public transportation and substitute bus travel for private cars,so as to reduce the per-capita road area in travel and achieve the purpose of relieving urban traffic congestion.Wherein,a representative concept is TOD model proposed by Peter Calthorpe in 1993,emphasizing high-density development around public transport stops,in order to increase the possibility of using public transport by people,reduce the number of traffic travel and travel distances,and improve the walking environment at the same time through mixed land use layout,small-scale neighbourhoods and other spatial approaches.However,in the traditional TOD model,only the spatial issues of traffic generation and arrival are taken into consideration from the perspective of the relationship between transportation and land use,and the problem of space in traffic 'process' is not considered adequately.As a spatial carrier for traffic,road plays a huge role in traffic 'process'.Therefore,we hope to propose measures favourable for public transport development from the perspective of urban road network planning,with a view to promoting the development of urban public transport and alleviating traffic congestion.One of the core contents of urban road network planning is to determine the road network structure,including layout structure,hierarchical structure and functional structure,but in our country,only hierarchical structural division of the road network is considered in actual work,urban road network is divided into four levels of expressway,trunk road,secondary trunk road and bypath,the road network layout and road network function are inadequately considered,and the approach of division is based on the travelling speed of a car,thus being a car-oriented hierarchy system in nature.Several methods on urban road network type and level division have been explored in foreign cities,including the considered network function(dominant mode of transportation),road location(space environment on both sides),transportation mode priority,etc.have important reference significance to construction of our road network hierarchy system.Hence,in the hope of learning from the advanced experience of foreign road network hierarchy systems,this paper explores the transit-oriented planning methods by taking road network hierarchy as a breakthrough.The conventional TOD model is out of the theoretical basis of the relationship between transportation and land use,measures in favour of public transportation are proposed from the perspective of land development,and this paper studies the transit-oriented road network hierarchy system,which is made on the theoretical basis of the relationship between road and traffic.Thus,this paper primarily explores the relationship between road and traffic and their interactive mechanisms.In terms of the relationship between road and traffic,this paper introduces the pair of philosophical categories of 'form and content' for a discussion by taking road as the 'form' of traffic and traffic as the 'content' of road,believes that the relationship therebetween is that 'traffic dominates road network structure,and road network structure reacts up on traffic',sorts out the history of road and traffic development in our country,and explains by the Law of Unity of Opposites,Law of Mutual Change of Quality and Quantity as well as Double Negation Law in Materialist Dialectics to demonstration the existence of the relationship of unity of opposites therebetween.The mutual interactive mechanisms of both are analyze on this basis,and the macro-mechanism(structural relationship)and the micro-mechanism(supply-demand relationship)interacted therebetween are summarized from 'structuralism' and transportation'properties'.The macro-mechanism exhibits in the relationship between road network structure and traffic structure and is specifically divided into road network layout structure,traffic distribution structure,road network hierarchical structure,traffic volume structure,road network functional structure and transportation mode structure;and the micro-mechanism embodies in the relationship between road network supply and traffic demand.Both macro-mechanism and micro-mechanism reflect the relationship of unity of opposites that 'traffic dominates road network structure,and road network structure reacts up on traffic'.Based on the relationship between roads and traffic and the corresponding interaction mechanism theory,this paper constructs a grading system for transit-oriented road network;it also puts forward the road network planning method per such system as well as a method of assessing transit-orientation of road network.As an inevitable traffic trend,public transport is an effective way of alleviating congestion and improving ecological environment.However,the current road network grading system in our country is still faced with multiple problems such as:network layout fails to adapt to traffic distribution,road network grading is not transit-oriented,dominant road network function is not clear,and the road network facilities fail to meet people's demand on travelling.Therefore,the transit-oriented system needs to be reconstructed.Drawing advanced experience from foreign countries,road network grading system is established in this paper,including "road network with bus priority,basic road network,community slow-speed road,and express parkway",on the basis of which "three-stage method" under such system is also put forward.The traditional "four-stage method" distributes traffic volume to each planned road,which is to "put the cart before the horse" from the perspective of relationship between road and traffic.The "three-stage method" proposed in this paper is consistent with the macro mechanism of interaction between road and traffic,and they are:"structure of traffic distribution determines road network layout,structure of traffic volume determines road network grading,structure of transportation means determines road network function".In the meantime,as for each type of transit-oriented road,the basic design characteristic,road intersection characteristic and facility supply characteristic are also put forward,in line with the micro mechanism of interaction between road and traffic.Moreover,assessment method for the transit-orientation in city road network system is also established from the perspectives of walkability,walking accessibility and public transportation-coverage.This method has some merits for it considers the influence of traffic "process" on people's choice of transport mode,including how road environment influences people's choice of travelling on foot,as well as how long they would like to spend walking to bus stations(common bus stops,bus stops in the bus priority road network and subway stations).Empirical research is carried out within the main urban area of Nanjing,on the theoretical basis of relationship between road and traffic and their interaction-mechanism,applying the established transit-oriented road network grading system,"three-stage" road network planning,and method of assessing road network transit-orientation,therefore to confirm the effectiveness,practicability,and transit orientation of the transit-oriented road network grading system.The road network in Nanjing is graded per traditional standard,thus is confronted with a series of problems:"road network is laid out without considering traffic distribution characteristic,road network is graded without referring to the structure of traffic volume,and the road network fails to meet traffic demands".By adopting the "three-stage method",the road network grading in Nanjing urban area is optimized into five types:expressway,road network with bus priority,basic road network,community slow-speed road,and express parkway.The optimized road network is assessed from walkability,walking accessibility and public transportation coverage.According to the assessment results,the optimized road network has better walkability,walking accessibility of bus stops has been greatly improved,and public transportation coverage has also been increased.According to the relationship between roads and traffic and the corresponding interaction mechanism theory,this paper establishes transit-oriented road network grading system,the"three-stage" method of road network planning,and the method of assessing transit-orientation in road network;the system is proved to be effective,practical and highly transit-oriented through the empirical research on Nanjing urban area.Nevertheless,there also exist some deficiencies,road network optimization fails to generate feedback on the-land utilization,case study fails to consider the newly built regions,and the transit-oriented extent is not assessed in real situation.In future study,efforts can be put on such aspects:using big data to accurately analyze city traffic structure;more detailed subdivision of road network grading types in various regions and cities;assessing the improvement of congestion before and after road network optimization. |
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