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 • 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.

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 Client’s Criteria and Preferences

5. 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) 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 Golden Rules

6. 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 13 ASSHTO Criteria

7. Factors: 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. Lane and Shoulder Widths

8. 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. Bridge Widths

9. Structural Capacity • HS-20 unless dealing with existing structures on local roads. • Check local practice • Handled by your bridge engineer

10. Factors: 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. Vertical Clearance

11. Anything is Fragile if You Have a Big Enough Hammer

12. 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” Grades

13. 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%. Cross Slope

14. Factors: 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 Superelevation

15. Factors: 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. Horizontal Alignment

16. Factors: 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. Vertical Alignment

17. Factors: 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. Stopping Sight Distance

18. Factors: 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. Horizontal Clearance

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. • Pedestrian mobility • ADA requirements

20. 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.

21. Urban ReconstructionGotcha • Utility Coordination • Right-of-Way Agreements • Permits • Maintenance of Traffic

22. Scope/Quality Reality Schedule Cost Better We Can Make It Better Engineer Client Faster Cheaper Done When Done Cost What It Costs Life