aashto subcommittee on design reducing engineering standards: good or bad? july 14, 2008 marc cote,...
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AASHTO Subcommittee on Design Reducing
Engineering Standards: Good or Bad?
July 14, 2008Marc Cote, P.E.(302) 760-2266
E-Mail: [email protected]
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Delaware is probably somewhat unique in that 91% of the roads in Delaware are state maintained.
Access to individual homes and streets in small subdivisions all have to be approved by my office in addition to the large subdivisions and commercial developments.
Reducing Engineering Standards: Good or Bad?
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Reducing Engineering Standards: Good or Bad?
Dedicating Streets to Public Use and State Maintenance
Built in accordance with DelDOT’s Subdivision Manual, Standard Construction Details, and Standards and Specifications.
Dedicating Streets to Public Use without State Maintenance
Built in accordance with DelDOT’s Subdivision Manual, Standard Construction Details, and Standards and Specifications.
but privately maintained.
Private Streets Not subject to our regulations, But DelDOT still suggests developers build to our standard.
AASHTO Subcommittee on Design
Subdivision Manual requires that subdivisions generating more than 400 vpd or 50 vph in the peak hour conduct a traffic impact study.
In addition to offsite improvements outlined in the TIS, the developer is required to improve his road frontage to meet the standards for the functional classification of the road.
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Reducing Engineering Standards: Good or Bad?
Department of Transportation
Functional Classification Map
Minimum Road Section
Freeway or ExpresswayPrincipal Arterial
12’ Lanes 10’ Shoulders
Minor ArterialMajor or Minor Collector
12’ Lanes8’ Shoulders
Local Road or Street (All roads other than Subdivision Streets
not shown)
11’ Lanes5’ Shoulders
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Reducing Engineering Standards: Good or Bad?
A completed design checklist must be filled out noting that all of the required information has been included on the plans, the design criteria is met, and is in accordance with DelDOT’s Standard Construction Details and Standards and Specifications.
Preliminary entrance plans shall include but are not limited to the following: traffic generation diagram, adjacent entrances, functional classification of adjacent roadway, layout of required auxiliary lanes, sight distance calculations.
AASHTO Subcommittee on Design
Developers are making substantial improvements to our road system and there are factors to consider:
Right of way impacts – local opposition Cost of improvements vs. the cost of
housing Improved roads over existing conditions If improvements are too extensive,
development doesn’t happen, improvement doesn’t happen
Support for economic development if it is planned
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AASHTO Subcommittee on Design
Want the best product for our citizens at the best cost
Need solution that works, doesn’t have to be the perfect solution
Improve the existing condition Don’t compromise on safety
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AASHTO Subcommittee on Design
Thomas Jefferson once said that:
“The first obligation of government is to provide for the safety of the people.”
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Areas for consideration Design speed Lane width Shoulder width Stopping sight distance on vertical and horizontal curves Vertical alignment Minimum and maximum grades Cross slopes Superelevation rate Horizontal clearance Vertical clearance Bridge width Structural capacity
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Reducing Engineering Standards: Good or Bad?
Standards to be maintained: Design speed Stopping sight distance on vertical and horizontal curves Vertical alignment Minimum and maximum grades Vertical clearance Bridge width Structural capacity Lane width
Standards considered for reduction: Shoulder width Cross slopes Superelevation rate Horizontal clearance Bridge width
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Reducing Engineering Standards: Good or Bad?
Development Related Improvements Requiring New Rights-of-Way
Gives DelDOT ability to acquire right-of-way for projects done by others (developers) to make improvements for the public’s benefit in the interest of safety.
AASHTO Subcommittee on Design
Design Element
DelDOT Std. Subdivision
DelDOT Mobility Friendly Design Guideline
Right-of –Way Width
60 feet 53-60’, Includes two 10’ lanes, 1’ curb offset, 6”curb on each side, one 7’ parking lane, and 10’ planting strip and 5’ sidewalk on each side
Pavement Width
32 feet 22’-29’, max. is two 10’ lanes, 1’ curb offset and 7’ parking lane
Travel Lane Width
11 feet 10 feet
Parking Lane Width
None specified
7 feet
Horz. Curve Radius
300 feet 167’ for unsigned curve, 90’ for signed curve used for traffic calming
Design Speed 30 mph 25 mph
Reducing Engineering Standards: Good or Bad?
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Design Element
DelDOT Std. Subdivision
DelDOT Mobility Friendly Design Guideline
Sidewalk warrants
Not required Required on both sides of street
Sidewalk widths None specified 5 feet minimum
Planting Buffer/Utility Strip
None specified 10 feet minimum
Alleys Alleys when lot width is <50’. 20’ r/w for 12’ lane
Corner radius None specified Local to local 25’
Maximum Cul-de-sac Length
500-100 feet depending on density of development
300 feet
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Design Element
DelDOT Std. Subdivision
DelDOT Mobility Friendly Design Guideline
Block Length None specified Between 200-500’. >500 requires a midblock crossing
Intersection Design
T-Intersection at 90 degrees
Roundabouts and two-way stops are preferred to signals
Minimum Driveway Spacing
200 feet, narrower lots call for shared driveways
50’, narrower lots require alleys or shared driveways
Minimum Driveway Width
12 feet standard, 8’-16’ depending on setback of garage
Tree/Obstacle Clearance
Clear zone of 2’ where a barrier curb is provided
Minimum of 3’ from back of curb to tree
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Background:
International Fire Code (IFC) states “fire access road shall have an unobstructed width of at least 20 feet.”
DelDOT Standards and Regulations for Subdivision Streets and Highway Access
Issue:
“Are existing 22’ wide streets too narrow to accommodate emergency vehicles?”
“Should a 24’ section be adopted by DelDOT?”
Wide Streets• Can encourage speeding• Increase impervious area• May reduce the livability of a residential street• Undermine perceived pedestrian/cyclist safety• Inconsistent with residential traffic patterns• Higher maintenance costs• Community requests for traffic calming measures
Source: Arlington County, VA residential streets presentation
Narrow Streets
• May inhibit emergency response• Can constrain snow plowing operations• May discourage free flowing traffic• Reduce vehicular speed• Can improve pedestrian and cyclist safety• Reduce maintenance• Less impervious area
“What Fire Departments Fear”
“Encourages High Speeds”
Subdivision StreetType 1-With Curb
(Emergency Vehicle and Standard Vehicle)
Sidewalk 5’
Grass Strip 3’
Curb 2’
Lane 11’
Lane 11’
Sidewalk 5’
Grass Strip 3’
Curb 2’
Gutter Pan11”
Gutter Pan11”
Fire Engine
9.5’ (mirror to mirror)
Pickup Truck
6.6’
Source: DelDOT Standards and Regulations for Subdivision Streets and State Highway Access Graphics by DelDOT Division of Planning
Existing DelDOT Standards
Subdivision StreetType 1-With Curb w/ 11’ Lanes
(Emergency Vehicle and Standard Vehicle)
Graphics by DelDOT Division of Planning
Top View
Top View
Subdivision StreetType 1-With Curb w/ 12’ Lanes
(Emergency Vehicle and Standard Vehicle)
Existing DelDOT Standardsw/parking both sides of street
Proposed DelDOT Standardsw/parking both sides of street
Top View
Graphics by DelDOT Division of Planning
Front View
Subdivision StreetType 1-With Curb w/ 11’ Lanes
(Emergency Vehicle and Standard Vehicle)
Subdivision StreetType 1-With Curb w/ 12’ Lanes
(Emergency Vehicle and Standard Vehicle)
Front View Existing DelDOT Standardsw/parking both sides of street
Proposed DelDOT Standardsw/parking both sides of street
Conclusion:The adoption of new 12’ lane requirements for subdivision streets that fall under the guidelines of the DelDOT Standards and Regulations for Subdivision Streets and Highway Access would enhance the ability of emergency vehicles to safely reach their intended destinations
Sidewalk 5’
Grass Strip 3’
Curb 2’
Lane 12’
Lane 12’
Sidewalk 5’
Grass Strip 3’
Curb 2’
Gutter Pan11”
Gutter Pan11”
Proposed DelDOT Standards
Graphics by DelDOT Division of Planning