School of Civil Engineering 孙仁娟 URBAN ROAD DESIGN Lecture 6- Design of At-grade Intersection

Terms An at-grade intersection is a junction at which two or more transport axes cross at the same level (or grade). At-grade intersections are an important part of an urban road. The efficiency, safety, speed, cost of operation, and capacity of the facility is a function of its design and how it operates. With areas of high or fast traffic, an at-grade intersection normally requires a traffic control device such as a stop sign, traffic light or railway to manage conflicting traffic.

Types One way classifies intersections as 3-way, 4- way, 5-way, 6-way, etc. depending on the number of road segments (arms) that come together at the intersection. Another way of classifying intersections is by traffic control: Uncontrolled intersections, Yield-controlled intersections, Stop-controlled intersections, Signal-controlled intersections, and traffic circles.

Objectives Describes several fundamental aspects of intersection design, including managing multimodal conflicts, sight distance and layout; and Provides general principles, considerations and design guidelines for key intersection components including curb return radii, channelized right turns, modern roundabouts, crosswalks, curb extensions, bicycle lanes and bus stops.

Clear Sight Triangles

Specified areas along intersection approaches, called clear sight triangles, should be free of obstructions that block a driver's view of potentially conflicting vehicles or pedestrians entering the traveled way. The determination of sight triangles at intersections varies by the target speed of the thoroughfares, type of traffic control at the intersection and type of vehicle movement. In urban areas, intersection corners are frequently entrances to buildings and are desirable locations for urban design features, landscaping and other streetside features. In designing walkable urban thoroughfares, the practitioner works in an interdisciplinary environment and has a responsibility to balance the desire for these streetside features with the provision of adequate sight distance, ensuring safety for all users. In urban areas, examples of objects that limit sight distance include vehicles in adjacent lanes, parked vehicles, bridge piers and abutments, large signs, poorly pruned trees, tall shrubs and hedges, walls, fences and buildings.

Traffic Conflicts Intersections, by their very nature, create conflicts between vehicles, pedestrians and bicyclists. The number of conflicts between different modes at three- and four-leg intersections are shown as:

Clarity—making it clear to drivers that pedestrians use the intersections and indicating to pedestrians where the best place is to cross; Predictability—drivers know where to expect pedestrians; Visibility—good sight distance and lighting so that pedestrians can clearly view oncoming traffic and be seen by approaching motorists; Short wait—providing reasonable wait times to cross the street at both unsignalized (via gaps created in traffic or two-stage crossings) and signalized intersections (via signal cycle length); Adequate crossing time at signalized intersections—the appropriate signal timing for all types of users to cross the street; Limited exposure—reducing conflict points where possible, reducing crossing distance and providing refuge islands when necessary; and Usable crossing—eliminating barriers and ensuring accessibility for all users.

Capacity The “capacity” of an intersection for any of its users (motor vehicles, pedestrians, bicyclists, transit vehicles) is the maximum rate of flow of that user type that can be accommodated through the intersection. They can be presented as Pedestrian Flow Capacity; Bicycle Flow Capacity; Motor Vehicle Capacity;

LOS Level-of-service is one measure of user satisfaction with an intersection. For all users, level-of-service is linked to average delay.

Geometric Design Elements Horizontal Alignment Ideally, streets should intersect as close to right angles as practical. Skewed intersections can reduce visibility of approaching motor vehicles and bicycles, require higher degrees of traffic control, require more pavement to facilitate turning vehicles, and require greater crossing distances for pedestrians.

Geometric Design Elements Vertical Alignment/Profile The major street and minor street profile influence the vertical alignment of an intersection.

Geometric Design Elements Pavement Cross-slope at Intersections

Geometric Design Elements Pavement Corner/Curb Radius

Auxiliary Lanes The design elements of three auxiliary lanes types are described in the following sections: left-turn lanes, right-turn lanes, and through lanes.

Left-turn lanes

Right-turn Lanes

Auxiliary Through Lane

Crosswalks Crosswalks are a critical element of intersection design. Crosswalks are essential for designating the appropriate path of travel for a pedestrian through the intersection. Crosswalks are defined by pavement markings, textured pavement, and colored pavement. Several techniques are available to shorten pedestrian crossings and for improving crosswalk visibility.

Planning and Design Specification: 《城市道路交叉口规划规范》 (GB ) (Code for Planning of Intersections on Urban Roads) 《城市道路交叉口设计规程》 (CJJ ) (Specification for design of intersections on urban roads)

Types 交叉口按城市大小与相交道路类型的分类

Types

Selection of the intersection type

Standards Design vehicle: Design period: Design speed: 50~70% Design lane width: 3.25m, 3.0m, 2.80m

Alignment and Profile

Pavement Corner/Curb Radius

Clear Sight Triangles

Auxiliary Lanes

Crosswalks

相交道路类型的确定相交道路类型的确定 交叉口类型的确定交叉口类型的确定 信号控制 交叉口 优先控制 交叉口 全无控制 交叉口 环形 交叉口 * 道路设计：交叉口夹角、交叉 口视距、进出口车道数及宽度、 转弯半径、分隔带设计等。 * 交通管理与控制方案设计： 相位相序、绿间隔、最小绿灯 时间、停车让行与减速让行及 行人自行车交通行为规范与 管理方案设计。 城市道路平面交叉口交通设计流程（新建道路）

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