AASHTO Subcommittee on Design Reducing Engineering Standards: Good or Bad?

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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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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AASHTO Subcommittee on Design7/14/08

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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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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.


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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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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Thomas Jefferson once said that:

“The first obligation of government is to provide for the safety of the people.”

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AASHTO Subcommittee on Design7/14/08

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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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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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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.


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



Design Element



Sidewalk warrants

Not required Required on both sides of street

Sidewalk widths None specified 5 feet minimumPlanting 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



Design Element



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



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’


Gutter Pan11”

Gutter Pan11”

Fire Engine9.5’ (mirror to mirror)

Pickup Truck6.6’

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


Graphics by DelDOT Division of Planning

Top View

Top View



Existing DelDOT Standardsw/parking both sides of street

Proposed DelDOT Standardsw/parking both sides of street

Top View


Front View





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’


Lane 12’

Lane 12’

Sidewalk 5’


Gutter Pan11”

Gutter Pan11”

Proposed DelDOT Standards


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AASHTO Subcommittee on Design Reducing Engineering Standards: Good or Bad?

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