school of civil engineering

School of Civil Engineering

孙仁娟2012.3.13

URBAN ROAD DESIGN

Lecture 5- Alignment and Profile

of Urban Road

Basic Terms

• The geometric design of highway deals with the positioning of the physical elements of the roadway according to standards and constraints.

• The basic objective in geometric design is to provide a smooth-flowing, crash-free facility.

• Geometric design can be broken into three main parts: horizontal alignment, vertical alignment (profile) and cross-section. Combined, they provide a three-dimensional layout for a roadway.

Basic Terms

• The alignment is the route of the road, defined as a series of horizontal tangents and curves.

• The profile is the vertical aspect of the road, including crest and sag curves, and the straight grades connecting them.

• The cross section shows the position and number of vehicle and bicycle lanes and sidewalks, along with their cross slope or bankng. Cross sections also show drainage features, pavement structure and other items outside the category of geometric design.

Standard & specification

• Urban roads are designed in conjunction with design guidelines and standards.

• The guidelines take into account speed, vehicle type, road grade (slope), view obstructions, and stopping distance.

• They are listed in

《城市道路工程技术标准》 (CJJ -2009)

《城市道路设计规范》（ CJJ 37-90)

Horizontal alignment---

• Horizontal alignment in urban road design consists of straight sections of road, known as tangents, connected by horizontal curves.

• The design of a horizontal curve entails the determination of a minimum radius (based on speed limit), curve length, and objects obstructing the view of the driver.

• If there is an object obstructing the view around a corner or curve, the engineer must work to ensure that drivers can see far enough to stop to avoid an accident or accelerate to join traffic.

Terminology

• R = Radius• PC = Point of Curvature (point at which the curve begins)• PT = Point of Tangent (point at which the curve ends)• PI = Point of Intersection (point at which the two tangents intersect)• T = Tangent Length• C = Long Chord Length (straight line between PC and PT)• L = Curve Length• M = Middle Ordinate, now known as HSO - Horizontal Sightline

Offset (distance from sight-obstructing object to the middle of the outside lane)

• e = Rate of Superelevation• f = Coefficient of Side Friction• u = Vehicle Speed

Horizontal alignment---

• Straight line

• Horizontal and circular curve

• Transition curve

• Sight distance

Straight line

• Highway

• Urban road

Horizontal and circular curve

• Minimum radius:• Highway

• Urban road


• General minimum radius?

• Ultimate minimum radius?

• Minimum radius without superelevation?


• Forces on banked roadways


• General minimum radius?

• Ultimate minimum radius?

• Minimum radius without superelevation?

• Equilibrium of forces

: sideway force coefficient

• Radius of curvature

h

2

h

2

h igR

vGGi

gR

GvGiFX

: superelevation

higR

v

G

X

2

)(127

2

hi

VR

hi



• Practical limits on superelevation– Climate– Constructability– Adjacent land use

• Side friction factor variations– Vehicle speed– Pavement texture– Tire condition


• Highway


• Urban road

• Dry condition: 0.4 ～ 0.8 ，• Wet asphalt: 0.25 ～ 0.40 ，• Ice condition: less than 0.2

hi


• Urban road

（一）缓和曲线的最小长度 1. 旅客感觉舒适

2. 超高渐变率适中

3. 行驶时间不过短

ss R

VL

3

(min) 0214.0

p

iBLs

(min)

2.1(min)

VLs

Horizontal and circular

Transition curve/easement curve

• Highway• 高速公路、一级公路、二级公路、三级公路的直线同小于不设超高的圆曲线最小半径径向

连接处，应设置回旋线。（ spiral curve ）• 四级公路的直线同小于不设超高的圆曲线最小半径相连接处，应设置超高、加宽过渡段。

2Alr

设计速度 (km/h) 120 100 80 60 40 30 20

缓和曲线最小长度 (m) 100 85 70 60 40 30 20


from the 2001 Caltrans Highway Design Manual


No Spiral

Spiral

from AASHTO’s A Policy on Geometric Design of Highways and Streets 2001

Sight distance

• Sight distance ( 视距） is how far a road user (usually a vehicle driver) can see before the line of sight is blocked by a hill crest, or an obstacle on the inside of a horizontal curve or intersection.

• Stopping sight distance （停车视距） is the distance traveled while the vehicle driver perceives a situation requiring a stop, realizes that stopping is necessary, applies the brake, and comes to a stop.

Sight distance

• Stopping sight distance （停车视距）

• 会车视距

S 停 =S1+S2+ （ m ）S

222101211· SSSSSS 会（ m）

Sight distance

• 全超车视距：

• 最小超车视距

4321 SSSSS 超

4323

2SSS

Passing sight distance

Sight distance

• Highway

Sight distance

• Urban road

Sight distance

• 1 、道路平面、纵断面上的停车视距应大于或等于表中规定值。积雪冰冻地区的停车视距宜适当增长。

• 2 、车行道上对向行驶的车辆有会车可能时，应采用会车视距，其值为表中停车视距的两倍。

• 3 、车行道上考虑有超车要求的路段，应采用超车视距，其值为表中停车视距的 5 倍。

• 4 、城市货运干道，应采用货车停车视距对相关路段进行检验。• 5 、对于凸形竖曲线和立交桥下凹形竖曲线等可能影响行车视距，危

及行车安全的地方，均需验算行车视距。验算时，物高为 0.1m ；目高在凸形竖曲线时为 1.2m ，在桥下凹形竖曲线时为 1.9m 。

• 6 、平曲线内侧的边坡、建筑物、树木等均不应妨碍视线。若平曲线内侧设置的人工构造物，或中央分隔带设置防眩设施时，应对视距予以检查与验算。

Profile

• The profile of a road consists of road slopes, called grades, connected by vertical curves. Vertical curves are used to provide a gradual change from one road slope to another, so that vehicles may smoothly navigate grade changes as they travel.

• Sag vertical curves are those that have a tangent slope at the end of the curve that is higher than that of the beginning of the curve. When driving on a road, a sag curve would appear as a valley, with the vehicle first going downhill before reaching the bottom of the curve and continuing uphill or level.

• Crest vertical curves are those that have a tangent slope at the end of the curve that is lower than that of the beginning of the curve. When driving on a crest curve, the road appears as a hill, with the vehicle first going uphill before reaching the top of the curve and continuing downhill.

Terminology• A = absolute value of the difference in grades (initial minus final, expressed

in percent)

• BVC = begin of vertical curve

• EVT = end of vertical tangent

• G1 = initial roadway (tangent)slope

• G2 = final roadway (tangent)slope

• = Height of eye above roadway, measured in meters or feet

• = Height of object above roadway, measured in meters or feet

• L = curve length (along the x-axis)

• PVI = point of vertical interception (intersection of initial and final grades)

• tangent elevation = elevation of a point along the initial tangent

• x = distance from PTC/BVC

• Y (offset) = vertical distance from the initial tangent to the curve

• Y’ = curve elevation = tangent elevation - offset

Profile• Objective:

– Determine elevation to ensure • Proper drainage• Acceptable level of safety

• Primary challenge– Transition between two grades– Vertical curves

G1 G2G1

G2

Crest Vertical Curve

Sag Vertical Curve

G1G2

PVI

PVTPVC

h2h1

L

SSD

Line of Sight

Crest Vertical Curves

G1 G2

PVI

PVTPVC

h2=0h1

L

Light Beam Distance (SSD)

headlight beam (diverging from LOS by β degrees)

Sag Vertical Curves

Profile

• Vertical grade and length of vertical slope

• Radius and length of vertical curve

Vertical grade and length of vertical slope

• Highway

• Urban road

• 表 6.3.2 最大纵坡

设计速度（ km/h) 100 80 60 50 40 30 20

最大纵坡推荐值 (%) 3 4 5 5.5 6 7 8

最大纵坡限制值 (%) 4 5 6 8 9


• Highway

• Urban road

Minimum length of the vertical slope1. Restraint the centrifugal force ( 限制离心力）一般情况下，单位车重的离心力（ F/G ）控制在 0.020 ～ 0.03 之间，则最小竖曲线长度为：

2. Restraint the running time （限制行程时间过程）

3.Meet the requirement of sight distance ( 满足视距要求 )

)()(127

2min m

G

FV

RL

)(2.1

36.3min m

VVtL

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Radius and length of vertical curve

• Highway

Radius and length of vertical curve

• Urban road

Design of horizontal and vertical alignment

• Match of horizontal and vertical curve

• Design procedures of alignment-horizontal

• Design procedures of alignment-vertical

Match of horizontal and vertical curve

• Urban road

Questions?

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