Title: CE 453 Highway Design Iowa State University Highway Design Criteria Overview
1 CE 453 Highway DesignIowa State UniversityHighway Design Criteria Overview
April 24 2006
David R. Dougherty P.E.
2 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
3 Design CriteriaWhere It All Starts
Level of Service Goals
Freeway Arterial Local etc.
Urban or Rural
Logical with respect to the topography anticipated operating speed adjacent land use and functional classification. Rule of thumb 5 mph over posted speed.
4 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 Clients Criteria and Preferences
5 Golden Rules
A. 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)
Design and Controls Criteria
Elements of Design
Cross Sections Elements
Local Roads and Streets
Collector Roads and Streets
Rural and Urban Arterials
Grade Separations and Interchanges
6 13 ASSHTO Criteria
Stopping Sight Distance
7 Lane and Shoulder Widths
Freeways w/ high percent trucks wider shoulders.
Freeways w/ 3 or more lanes per direction wider inside shoulders.
Interchange ramps typically wider because of turning roadway nature but shoulders are narrower.
Average Daily Traffic
Number of Lanes
Geometry Turning Roadways
Provisions for Passing Stalled Vehicles
8 Bridge Widths
Average Daily Traffic
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.
9 Structural Capacity
HS-20 unless dealing with existing structures on local roads.
Check local practice
Handled by your bridge engineer
10 Vertical Clearance
What is under
Structure type might influence clearance
Measure at edge of travel way or usable shoulder edge at critical clearance point
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.
11 Anything is Fragile if You Have a Big Enough Hammer 12 Grades
Facility Type Speed
Drainage Curb and Gutter or Shoulder
Length of Grade
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
13 Cross Slope
Number of Lanes
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.
Centripetal Acceleration e f or both
Rural or Urban
Slow moving vehicles
AASHTO now says spirals not required. Iowa is keeping them
Design values for f include substantial margin of safety
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
15 Horizontal Alignment
Minimum Length of Curve
Coordinate Vertical and Horizontal Alignments
Dont 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.
16 Vertical Alignment
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
Dont 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.
17 Stopping Sight Distance
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
Intersection sight triangles
Decision sight distance situations requiring avoidance maneuvers. Reaction maneuver time varies from 3 seconds to 14.5 seconds.
Horizontal curves with sight obstructions.
18 Horizontal Clearance
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
Shoulders and Auxiliary Lanes count as part of clear zone.
Steeper than 31 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.
19 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.
20 Exceptions to the Rules
3R Projects Resurfacing Restoration and Rehabilitation
Context Sensitive Design
Crash and Safety Studies Cost/Benefit
Florida Handout Help navigate the Green Book (Based on 2001). See your local jurisdiction for design exception procedures.
21 Urban ReconstructionGotcha
Maintenance of Traffic
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