CE 453 Highway DesignIowa State University

Highway Design Criteria Overview

April 24, 2006

David R. Dougherty, P.E.

Some Basics• Highway Design is a Science and an Art• Need to understand the context our designs are within and how

criteria translates into driver expectancy/behavior and vehicle performance/behavior.

• Balance safety, cost, mobility, community values, environmental, politics, liability, sustainable development, etc.

• Our job is to protect the health, safety and welfare of the public

“A skilled highway designer or design team understands the human characteristics of drivers, bicyclists, and pedestrians as they relate to the driving task. A skilled designer also has knowledge of the physical and operational characteristics of vehicles. Finally, and perhaps more importantly, the skilled highway designer is able to translate this knowledge to the actual design of the highway.” A Guide for Achieving Flexibility in Highway Design, May 2004, by AASHTO

Design CriteriaWhere It All Starts

• Traffic Volumes• Level of Service Goals• Design Vehicle• Functional Classification

– Freeway, Arterial, Local, etc.– Urban or Rural– Terrain

• Design Speed– Logical with respect to the topography, anticipated operating

speed, adjacent land use and functional classification. Rule of thumb = 5 mph over posted speed.

Highway Design Criteria

• 13 Basic Design Criteria Based on AASHTO• Additional criteria derived from basic criteria• AASHTO provides guidance, range of values

– Local jurisdictions might be more stringent or might use lower range values (DOT, County, City)

– Local jurisdictions establish standards based on AASHTO guidance

• Know Your Client’s Criteria and Preferences

Golden RulesA. Define the Facility Type

B. Define the Design Speed

Green Book Organized by Functional Classification (Page references in this presentation to 2001 AASHTO Green Book)

Green Book

• Highway Functions• Design and Controls Criteria• Elements of Design• Cross Sections Elements• Local Roads and Streets• Collector Roads and Streets• Rural and Urban Arterials• Freeways• Intersections• Grade Separations and

Interchanges

13 ASSHTO Criteria

• Design Speed• Lane Widths• Shoulder Widths• Bridge Widths• Structural Capacity• Vertical Clearance• Grades

• Cross Slope• Superelevation• Horizontal Alignment• Vertical Alignment• Stopping Sight

Distance• Horizontal Clearance

Lane and Shoulder WidthsFactors:• Facility Type• Average Daily Traffic• Number of Lanes• Auxiliary Lanes/Ramps• Design Speed• Traffic Mix• Geometry – Turning Roadways• Provisions for Passing Stalled

Vehicles• Drainage Needs• Sight Distance• Local Practice

Examples:– Freeways w/ high percent

trucks = wider shoulders.– Freeways w/ 3 or more lanes

per direction = wider inside shoulders.

– Curb offsets can vary 1 to 2.5 feet.

– Low Speed, Low Volume roadways allow narrower lanes.

– Interchange ramps typically wider because of turning roadway nature, but shoulders are narrower.

Bridge Widths

Factors:• Facility Type• Bridge Length• Average Daily Traffic• Geometric

Considerations• Local Practice

Examples:– Typically match roadway

approach width.– Long bridges might have

less than full width shoulders.

– Farm to Market: large equipment.

– Curved Flyovers for interchanges – stopping sight distance.

Structural Capacity

• HS-20 unless dealing with existing structures on local roads.

• Check local practice

• Handled by your bridge engineer

Vertical ClearanceFactors:• Facility Type• What is under• Structure type might

influence clearance• Measure at edge of travel

way or usable shoulder edge at critical clearance point

Examples:– 16 feet is typical.– Most local agencies add 6”

for future overlays or future bridge widening.

– Higher clearances for “fragile” structures (sign bridge, ped bridge).

– Possible lower clearances for local roadways.

– 23 feet for clearance over railroads. Rail bridge type also influences clearance.

– Utility company criteria.

Anything is Fragile if You Have a Big Enough Hammer

Grades

Factors:• Facility Type & Speed• Terrain• Drainage – Curb and

Gutter or Shoulder• Length of Grade• ADA• Intersections

Examples:– Short segments may

have steeper grades.– Most local agencies

require 0.4% to 0.5% minimum grades for curb and gutter – AASHTO says 0.3%.

– Stopping at intersections and the “through roadway”

Cross Slope

Factors:• Facility Type• Number of Lanes• Drainage• Local Practice

Examples:– 2% is typical minimum,

AASHTO allows 1.5%.– 1.5% minimum in median

turn lanes. Some agencies allow 1% in left turn lane areas.

– 3 lanes and wider: outside lanes typically 3%.

– Shoulders typically 4%.

SuperelevationFactors:• Centripetal Acceleration: e, f or

both• Facility Type• Design Speed• Region/Area

– Snow/Ice– Terrain– Rural or Urban– Slow moving vehicles

• Local Practice • AASHTO now says spirals not

required. Iowa is keeping them• Design values for f include

substantial margin of safety

Examples:– Max super tied to region. Lower

for areas with ice and snow.– Low Speed Urban: Acceptable of

more Gs. Typically do not want to super in urban setting.

– Watch out for trapping drainage in transition areas.

– Length of transition depends on maximum rate of change of grade of outside edge: depends on number of lanes and rotation point.

– Location of transition relative to PC/PT varies by jurisdiction

– New AASHTO tables for 2004

Horizontal AlignmentFactors:• Facility Type• Design Speed• Minimum Length of

Curve• Sight Distance• Coordinate Vertical

and Horizontal Alignments

Examples:– Don’t hide changes in

horizontal behind crest verticals: Pg 283 AASHTO .

– Intersection sight distances: hard to judge distance on curve.

– No broken back curves (two consecutive curves in same direction separated by short tangent.

– Reverse curves: consider super transition.

– Rules for compound curves: Pg 205 AASHTO.

Vertical AlignmentFactors:• Design Speed• Driven by stopping sight

distance for crests.• Object height: previous

was 6”, now 2’. Iowa still prefers 6”.

• Sag curves driven by headlight sight distance, but also consider:– G force– Drainage

Examples:– Don’t hide changes ahead,

behind crest curves.– Watch out for long flat sag

and crest curves with curb and gutter – remember the minimum grade requirements for drainage.

– Consider passing sight distance and decision sight distance.

Stopping Sight DistanceFactors:• Design Speed• Height of Eye – 3.5’• Height of Object – 2’• Reaction Time – 2.5

seconds• Deceleration Rate – 11.2

ft/s2

• Passing Sight Distance• Decision Sight Distance• Based on Passenger Car• Adjustments for Grade

Examples:– Intersection sight triangles– Intersection maneuvers– Decision sight distance:

situations requiring avoidance maneuvers. Reaction & maneuver time varies from 3 seconds to 14.5 seconds.

– Horizontal curves with sight obstructions.

Horizontal ClearanceFactors:• Facility Type

• Design Speed

• Average Daily Traffic

• Offsets to objects from edge of travel lane

• Clear Zone = Unobstructed, relatively flat area provided beyond the edge of the traveled way for the recovery of errant vehicles.

• AASHTO Roadside Design Guide

• Local Practice

Examples:– Shoulders and Auxiliary Lanes

count as part of clear zone.– Side Slopes:

• 4:1 Recoverable• 3:1 Traversable• Steeper than 3:1 Non-

traversable – outside clear zone or shield

– Beak-away objects: meet horizontal clearance requirements, but can be within clear zone.

– No curbs or sloped curbs > 45 mph.

Other Criteria• Level of Service – Traffic Operations• Turn Lanes and Tapers• Design Vehicle for Turning Roadways• Construction Staging (How many lanes need to stay

open)• Border Area for urban projects: room between curb and

right-of-way for utilities, landscaping, sidewalk, snow storage, etc.

• Pedestrian mobility• ADA requirements

Exceptions to the Rules

• 3R Projects – Resurfacing, Restoration and Rehabilitation

• Context Sensitive Design• Design Exceptions• Crash and Safety Studies – Cost/Benefit• Florida Handout – Help navigate the Green Book

(Based on 2001). See your local jurisdiction for design exception procedures.

Urban ReconstructionGotcha

• Utility Coordination

• Right-of-Way Agreements

• Permits

• Maintenance of Traffic

LifeScope/Quality

Reality

CostSchedule

Better

Client

Faster Cheaper

Engineer

We Can Make It Better

Done When Done Cost What It Costs