GEOMETRIC HIGHWAY DESIGN

CE327: Transportation Systems Engineering

National Design Standards

American Association of State Highway and Transportation Officials (AASHTO) Representatives from State DOTs and FHWA

Technical Committees develop design standards

A Policy on Geometric Design of Highways and Streets

Guide for Design of Pavement Structures

AASHTO LRFD Bridge Design Specifications

Regional/Local

New York State Department of Transportation (DOT) Highway Design Manual

https://www.dot.ny.gov/divisions/engineering/design/dqab/hdm

Bridge Manual https://www.dot.ny.gov/divisions/engineering/structures/manuals/bridge-

manual-usc

Comprehensive Pavement Design Manual https://www.dot.ny.gov/divisions/engineering/design/dqab/cpdm

New York City DOT

Standard Highway Specifications http://www.nyc.gov/html/dot/downloads/pdf/standard%20_highway_specs

_vol%201.pdf

Street Design Manual http://www.nyc.gov/html/dot/html/about/streetdesignmanual.shtml

Highway Functional Classifications NYSDOT Highway Design Manual

Interstates Interstate highways are freeways on the interstate highway system. Generally, they are interregional high-speed, high-volume, divided facilities with complete control of access and are functionally classified as principal arterials.

Other Freeways

Other freeways are local, intraregional and interregional high-speed, divided, high-volume facilities with complete control of access. Most other freeways have been classified as principal arterials. Expressways are divided highways for through traffic with full or partial control of access and generally with grade separations at major crossroads.

Rural Arterials A major part of the rural highway system consists of rural arterials, which range from two-lane roadways to multilane, divided, controlled-access facilities. Generally, they are high-speed roadways for travel between major points.

Urban Arterials Urban arterials generally carry large traffic volumes within and through urban areas. They vary from multilane, divided, controlled-access facilities to two-lane streets. They serve major areas of activity, carrying a high mileage proportion of an area's traffic on a small proportion of the area's lane

Rural Collectors Rural collectors are two-lane roadways connecting roadways of higher classification, larger towns, and smaller communities. They link local traffic generators with rural areas.

Urban Collectors Urban collector streets link neighborhoods or areas of homogeneous land use with arterial streets. They serve the dual function of land access and traffic circulation.

Local Rural Roads Local rural roads are primarily town and county roads. Their primary purpose is access to the abutting property. They constitute a high proportion of the highway mileage but service a low proportion of the traffic volume.

Local Urban Streets Local urban streets are primarily village and city streets. Their primary purpose is access to abutting property.

AASHTO Vehicle Classifications

Design Vehicle = largest vehicle likely to use a highway with considerable frequency

Geometric Roadway Design 15 Design Vehicles Based on real configurations

Passenger Car Trucks Buses Recreational Vehicles

Bridge Design 8 basic design vehicles (some with variable axle spacings) Vehicles do not represent real truck configurations; rather represent

vehicles that would theoretically maximize bridge damage

AASHTO Design Vehicles Roadway Geometrics Passenger/Recreational Vehicles

Passenger Car

Motor Home

Passenger Car and Camper Trailer

Passenger Car and Boat Trailer

Motor Home and Boat Trailer

AASHTO Design Vehicles Roadway Geometrics Buses

Intercity Bus (BUS40)

Intercity Bus (BUS45)

City Transit Bus

Articulated Bus

Conventional School Bus

Large School Bus

AASHTO Design Vehicles Roadway Geometrics Trucks

Single Unit Truck

Intermediate Semitrailer (WB40)

Intermediate Semitrailer (WB50)

Interstate Semitrailer (WB62)

Interstate Semitrailer (WB65/WB67)

AASHTO Design Vehicles Roadway Geometrics Combination Trucks

DoubleTrailer

Combination

TripleTrailer

Combination

Turnpike Double

Combination

Turning Radius

AASHTO Design Vehicles - Bridges

Truck Size and Weight Limits

Federal Max gross vehicle weight (GVW) = 80 kip

Max single axle weight = 20 kip

Max tandem axle weight = 34 kip

Max width = 102 in

Max single trailer length = 53

State Limits on state networks vary

Some states can allow larger vehicles on National Highway Network: grandfathered limits were frozen by the Intermodal Surface Transportation Efficiency Act of 1991 (ISTEA)

NYSDOT - 17 Critical Design Elements

Design Speed

Lane Width

Shoulder Width

Bridge Roadway Width

Grade

Horizontal Curvature

Superelevation

Stopping Sight Distance

Horizontal Clearance

Vertical Clearance

Travel Lane Cross Slope

Rollover

Structural Capacity

Level of Service

Control of Access

Pedestrian Accommodation

Median Width

Specific Design Criteria

Chapter 2.7 of NYSDOT Highway Design Manual

Engineering judgment may be used when design criteria may not be met May create a liability concern

Requires careful documentation of reasoning

Cross Section Elements

Shoulders

Provide area for vehicle to stop

Provide lateral support to pavement

May be used by bicycles (rural and collector)

Graded Shoulder: total width

Usable Shoulder: area that can accommodate parked vehicles (same as graded if slope < 4:1)

Cross-Slope and Rollover

Cross-slope: Transverse slope of a travel lane

Rollover: Measure of the difference in cross slope between two adjacent highway lanes or a highway lane and an adjacent shoulder

Horizontal Clearance

Vertical Clearance

Highway Alignment

Least costly alignment follows natural topography Vertical curves connect straight highway grades

Horizontal curves connect straight sections of roadway

Horizontal and vertical elements must be balanced

Design should follow consistent standards

Horizontal Curves

Low Speed Curve Design

Design speed < 15 mph Determine design vehicle

Determine curve type

Simple curve (arc of a circular curve)

Simple curve with taper

3-centered compound curve

Use AASHTO Design Tables to determine minimum pavement edge (see handout)

Simple Curve

Simple Curve with Taper

High Speed Curve

Use minimum curve radius formula

However, must provide additional pavement width for maneuvering

Physics and Geometry Review

Newtons Second Law

Newtons Second Law

x

y

+

+

-

-

z +

-

Centripetal Force

Centripetal Acceleration

Centripetal Force

R

u W

Minimum Radius of a Circular Curve

W

fs

Centrifugal Force

Breaking Distance

W

f

Centrifugal Force

Stopping Sight Distance

Perception reaction time varies between users

Vision

Age

Distance from stimulus

AASHTO Design Value = 2.5 sec

Circumference of a Circle

= 2

Minimum Radius: Unobstructed Curve

Minimum Radius: Obstructed Curve

Simple Curve Design

Compound Curve

Two or more simple curves in succession Turning in same direction

Share a common tangent point

Used to obtain desirable shape for alignment Intersections

Interchanges

Highways in difficult topography

Max difference in radii Recommended: 1.75:1

In practice: 2:1

Compound Curve

Reverse Curve

Two simple curves with equal radii turning in opposite directions

Share a common tangent point

Generally used to change the alignment of a highway

Not recommended because sudden change in curvature difficult to maneuver; preferred alignment would include tangent connector

Vertical Curves

Vertical Curves

Parabolic curve that connect two highway grades

Minimum Curve Length

Minimum Curve Length

Stopping Sight Distance Distance required for vehicle to recognize a signal or

obstruction, apply brakes, and come to a stop

Passing Sight Distance Distance required for a vehicle to safely pass another vehicle

and return to its lane

Crest Vertical Curves, S>L

Criterion: Minimum stopping sight distance

Crest Vertical Curves, S

Sag Vertical Curve

Criteria: Headlight Stopping Sight Distance

S

Sag Vertical Curve

Criteria: Comfort Interaction of gravity and

centrifugal force

Criteria: Appearance

Criteria: Drainage Must be considered when

road is curbed

Minimum slope of . 3 percent usually provided within 50 ft of lowest point on curve

Elevation

Crest Vertical Curve Design Example

G1 = +4

G2 = -2

PVI = 25+00.0

PVI Elevation = 150 ft

u = 70 mph

PRT = 2.5 sec

A = 11.2 ft/sec2

Stations and Elevations on Curve

Station Distance from BVC Tangent Elevation Offset Curve Elevation

17.0 62.8 118.0 -0.1 117.9

18.0 162.8 122.0 -0.5 121.5

19.0 262.8 126.0 -1.2 124.8

20.0 362.8 130.0 -2.3 127.7

21.0 462.8 134.0 -3.7 130.3

22.0 562.8 138.0 -5.5 132.5

23.0 662.8 142.0 -7.6 134.4

24.0 762.8 146.0 -10.1 135.9

25.0 862.8 150.0 -12.9 137.1

26.0 962.8 154.0 -16.1 137.9

27.0 1062.8 158.0 -19.6 138.4

28.0 1162.8 162.0 -23.5 138.5

29.0 1262.8 166.0 -27.7 138.3

30.0 1362.8 170.0 -32.3 137.7

31.0 1462.8 174.0 -37.2 136.8

32.0 1562.8 178.0 -42.5 135.5

33.0 1662.8 182.0 -48.1 133.9

Sag Vertical Curve Design Example

G1 = -4

G2 = +2

PVI = 25+00.0

PVI Elevation = 150 ft

u = 70 mph

PRT = 2.5 sec

A = 11.2 ft/sec2

Stations and Elevations on Curve

Station Distance from BVC Tangent Elevation Offset Curve Elevation

20.0 91.1 170.0 0.2 170.2

21.0 191.1 166.0 0.9 166.9

22.0 291.1 162.0 2.2 164.2

23.0 391.1 158.0 3.9 161.9

24.0 491.1 154.0 6.1 160.1

25.0 591.1 150.0 8.9 158.9

26.0 691.1 146.0 12.1 158.1

27.0 791.1 142.0 15.9 157.9

28.0 891.1 138.0 20.2 158.2

29.0 991.1 134.0 24.9 158.9

30.0 1091.1 130.0 30.2 160.2