Assessing Street and Parking Design Standards to Reduce
Excess Impervious Cover in New Hampshire and Massachusetts
Small MS4 Permit Technical Support Document, April 2011
Draft NPDES Permits require evaluation of local
street and parking lot design standards
The draft NPDES Small MS4 permits for New Hampshire
and North Coastal Massachusetts require permittees to
evaluate and report on local street design and parking
requirements that affect the creation of impervious
cover. This assessment will be used to determine if
design standards need to be revised to support the
application of Low Impact Development (LID)
techniques. Recommendations and a schedule for
changing any relevant standards and policies need to be
incorporated into the Stormwater Management Program
(SWMP), with status updated in annual reports. This
requirement is detailed in the draft permit Section 2.3.6.6
for New Hampshire and Section 2.4.6.7 for North Coastal
Massachusetts, respectively.
Why evaluate current standards?
Roads and parking lots are a significant component of the
urban landscape, and often constitute the majority of
impervious area in a given the watershed. In many
communities, the current standards guiding road design
and parking lot layout were established decades ago with
little consideration of potential impacts to pedestrians or
the local environment. Consequently, outdated zoning by-
laws, subdivision regulations, and road standards may not
only promote excessive impervious cover (Figure 1), but
they may effectively prohibit the application of many LID
practices (Figure 2). Even where variances and special
permitting procedures allow for design alternatives, these
additional steps can be time-consuming and unpredictable;
and therefore, unattractive to developers.
Figure 1. Unnecessarily wide cul-de-sacs and residential roads
generate additional Stormwater runoff, create un-friendly
pedestrian environments, and increase overall construction
costs.
Figure 2. (A) Example of narrow residential road with a bio-
swale, utilities, and single-sided sidewalk in Duxbury, MA.
(B) Use of pervious pavers and bioretention practices in the
landscape islands in spillover parking lot in Wilmington, MA.
What design factors lead to excess imperviousness?
At a minimum, the following street and parking standards
should be evaluated to determine if they are contributing
to the unnecessary generation of surplus impervious cover
from new construction or redevelopment projects:
Local street design:
• Residential roadway pavement widths—
pavement widths should be set based on the number
of homes served, anticipated vehicle usage, and on-
street parking requirements. Establish minimum
and maximum standards to meet these needs while
avoiding excessively wide streets.
• Non-residential and mixed use roadway
pavement widths—pavement widths should be set
based on traffic volumes, types of vehicles, parking,
and pedestrian requirements, which often require
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more complex analysis. Provide flexibility to
accommodate this analysis, particularly in mixed
use/and or Traditional Neighborhood Districts.
• Road right-of-way (ROW) widths and usage—
large ROW's can increase the overall area disturbed
during development. Allow for flexibility in
widths, where appropriate, and for the placement of
utilities below the paved portion of the roadway to
allow for the use of roadside swales or other
stormwater practices.
• Building frontage and setback requirements—
residential road length is often determined by the
required frontage distance for individual lots.
• Turnarounds for dead end streets—road layouts
that reduce the number of dead end streets are
preferable. Provide options for turnaround designs
(cul-de-sacs, loop-de-lanes, T-shaped, etc). To
minimize impervious cover, maximum paved
diameters for cul-de-sacs should be based on the
required turning radius for emergency response
vehicles and should also allow for landscaped
islands (Figure 3).
• Sidewalks—consider pedestrian preferences when
designing sidewalks, rather than the blanket
application of a requirement for the placement of
sidewalks on both sides of the roadway. Allow for
sidewalks to be paved with pervious materials.
• Driveways—driveway dimensions can be
minimized through reduced minimum widths and
front yard setbacks. Standards should allow for
pervious driveway materials, allow "two-track"
designs (i.e., paved tire track with pervious median),
and prohibit direct rooftop discharge on to
impervious driveway surfaces. Shared driveways
should be allowed and sample agreements should be
provided by the municipality.
A
M R-30'MIN.
18' WIN. D=?IVE
AISLES (SEE NOTE)
B
LANDSCAPED/
BIORETENTION
AREA
SEE TYP.
OSS SFCTION
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Parking lot standards:
• Parking ratios—the number of required parking
spaces is often based on parking demand studies
that are not locally applicable, expressed only as a
minimum standard for the worst case scenario, and
often result in an oversupply of parking. In these
cases, communities should be comfortable
establishing maximum parking requirements at
current minimum standards and new minimums set
~ 1/3 below these revised maximums (see Table 1).
• Off-street and on-site parking—in urban and
village centers, consider dramatic changes to the
typical parking demand requirements to provide
flexibility in design. Consider revising off-site
distance limits, as well as the amount of public
parking allowed to help satisfy private parking
requirements.
• Credits for shared parking and mass transit—
allow for reductions in parking requirements for
shared parking arrangements, parking garages, and
in areas where mass transit is accessible. Provide
model shared parking contracts.
• Stall and driving aisle dimensions—avoid
requiring excessively wide stalls and driving aisles.
Standard stall dimensions can be as small as 9 ft x
18 ft. Driving aisle widths should be based on
orientation of parking stalls and whether traffic flow
is single or two-way.
• Pervious parking—allow the use of structural
permeable pavement options where appropriate;
allow spillover parking (or parking above minimum
requirement) to be pervious.
• Landscape requirements—landscape islands and
borders are often required for traffic flow and
screening purposes. The total landscaped area is
often a calculated based on the number of parking
spaces or amount of total impervious cover.
Vegetated stormwater practices should be
incorporated into these features; the amount of
required landscaping should be sufficient to meet
tree canopy/shade requirements and adequate for
long-term tree survival.
A more detailed discussion of preferred parking lot
design, planning options, and a model parking by-law can
be found online at the MA Smart Growth/Smart Energy
Toolkit www.mass.gov/envir/smart growth toolkit/.
Figure 3. (A) Existing design details may require updating (B)
to accompany revised street and parking requirements, such as
in this revised cul-de-sac detail for Attleboro, MA that
incorporates a reduced paved radius and a central
bioretention/landscaped island.
Other important site design requirements
In addition, a number of other site design factors can have
a significant impact on the amount of impervious cover
created at a site and whether it is connected or
disconnected to the storm drain system. Examples
include:
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• Allowing open space residential development (i.e.,
conservation design or low impact development) that
provides for reduced setbacks and smaller lot sizes as
"by-right" without additional permitting;
• Restricting the percentages of impervious and turf
cover on individual lots;
• Allowing for open-section (i.e., curb-less) roads
through flexibility in curbing requirements;
• Allowing for temporary ponding of stormwater on
residential lots;
• Requiring the routing of rooftop runoff to pervious
areas, dry wells, or other devices to promote
infiltration and/or stormwater reuse;
• Requiring integration of landscaping and stormwater
management requirements.
Table 1. Example of suggested parking requirements per 1,000
sq ft of Gross Floor Space (excerpt from the Smart Parking By-
law, MA Smart Growth/Smart Energy Toolkit)
Land Use
Bank
Large Scale Retail
General Office Building
Medical Building
Nursing Home
Restaurants
Shopping Centers
Bed and Breakfast
Personal Services
Churches and Places of
Worship
Museums and Libraries
Public and Private
Educational Institutions
Maximum
3
4
4
8
3
10
4
1.2 spaces/guest
room or suite
3
1 space/3 seats in
service portion of
the building
2
1 space/3 seats in
the classroom
Minimum
2
2
2
2
2
6
3
1 space/guest
room or suite
2
1 sp/5 seats in
service portion
of building
1
1 sp/5 seats in
classroom
Challenges to updating design standards
Consider including representatives of local planning
boards, water suppliers and other utilities, transportation,
public works, emergency response, school
superintendents; and the development community in the
review process to help address some of the following
concerns related to street design and parking standards:
• Safety concerns (i.e., fire, school bus) for setbacks,
turnarounds, permeable pavers, and road widths;
• Utility installation and maintenance in public ROWs;
• Snow removal requirements for parking lots,
landscape islands, and turnarounds; and
• Retail parking demands set by financial institutions
for minimum parking requirements.
How do I report on our assessment of local
Within two years of the effective permit, permittees must
have developed a report on the assessing current street
design, parking lot guidelines, and other local
requirements that affect the creation of impervious cover.
This report should clearly indicate which design
standards promote excess impervious cover and any
recommended changes.
There are a number of checklists, self-audits, and model
bylaws available to assist communities in evaluating street
and parking standards including the Codes and Ordinance
Worksheet from the Center for Watershed Protection
(www.cwp.org) and the LID Local Codes Checklist from
the Massachusetts Planning Commission
(www.mapc.org/LID). Table 2 provides a simplified
checklist that can be used to help satisfy SWMP and
annual reporting requirements. A narrative describing any
recommended (or completed) changes must also be
included.
Within three years, permittees must also have developed a
report assessing regulatory barriers to implementing
structural LID practices (e.g., green roofs, infiltration
practices, and water harvesting devices).
It may be advantageous to conduct and report on both
assessments concurrently.
Other References
CWP. 1998. Better Site Design: A handbook for
changing development rules in your community
www.cwp.org
EPA. 2006. Parking Spaces/Community Places: Finding
the balance with smart growth solutions.
www .epa. gov/smartgrowth/pdf/EP AParking Space s06.
rjdf
American Planning Association, Massachusetts and Home
Builders Association of Massachusetts. October
2010. Sustainable Neighborhood Road Design: A
guidebook for Massachusetts cities and towns.
www.apa-ma.org/resources/publications/nrb-
guidebook
New Hampshire Department of Environmental Services.
2008. Innovative Land Use Planning Techniques: A
handbook for sustainable development.
http://des.nh.gov/organization/divisions/water/wmb/re
pp/documents/ilupt_complete_handbook.pdf
Rhode Island Department of Environmental Management.
Rhode Island Community LID Site Planning and
Design Guidance Document. 2011.
Maryland Governor's Office of Smart Growth. Driving
Urban Environments: Smart growth parking best
practices.
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Table 2. Checklist for evaluating street and parking standards (adapted from CWP Codes and Ordinances Worksheet and MAPC LID Checklist*)
STREETS
ft
1.1. Is the minimum pavement width for low traffic residential roads (<500 average daily trips) between 18-22 ft?
nYes n No n No standard n Don't know Action: n Leave as is n To be revised
1.2. Can parking lanes serve as traffic lanes in higher density areas?
n Yes n No n No standard n Don't know
Action: n Leave as is n To be revised
4*
4*
1.3. Are narrower pavement widths allowed on road sections were there are no houses, buildings, intersections, or on-street parking spaces?
nYes nNo n No standard n Don't know Action: n Leave as is n To be revised
1.4. Are reductions in frontage distances allowable where appropriate (i.e., open space developments, around cul-de-sacs, and along outside
sideline of curved streets) to minimize street length?
nYes
nNo
n No standard n Don't know
Action: n Leave as is n To be revised
1.5. Can permeable paving be used for residential roads, shoulders, and parking lanes?
n Yes n No n No standard n Don't know
Action: n Leave as is n To be revised
If
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2.1. Are minimum ROW widths less than 45 ft for a residential street?
n Yes n No n No standard n Don't know
ft
Action: n Leave as is n To be revised
2.2. Can utilities be placed below the paved section of the ROW?
n Yes n No n No standard n Don't know
Action: n Leave as is n To be revised
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3.1. Are landscaped/bioretention islands required in the center of cul-de-sacs?
n Yes n No n No standard n Don't know
Action:
n Leave as is n To be revised
3.2. Is the minimum required radius for cul-de-sacs less than 35 ft?
.ft
nYes
nNo
n No standard n Don't know
Action:
n Leave as is n To be revised
3.3. Are alternatives to cul-de-sacs such as "hammerheads" allowed for permanent turnarounds?
n Yes n No n No standard n Don't know
Action:
n Leave as is n To be revised
3.4. Are alternative road layouts such as one-way loops encouraged to eliminate dead end streets?
n Yes n No n No standard n Don't know
Action:
n Leave as is n To be revised
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4. Sidewalks
5. Driveways
4.1. Are sidewalks always required on both sides of residential streets?
nYes nNo nNo standard n Don't know Action: n Leave as is n To be revised
4.2. Is permeable paving allowed for sidewalks?
nYes nNo nNo standard n Don't know Action: n Leave as is n To be revised
4.3. Are alternative pedestrian pathway layouts allowed, rather than placement in road ROW?
nYes nNo nNo standard n Don't know Action: n Leave as is n To be revised
5.1. Are reductions in setback distances allowable where appropriate to minimize driveway lengths?
nYes nNo nNo standard n Don't know Action: n Leave as is n To be revised
5.2. Is the minimum driveway width 9 feet or less (single lane) or 18 feet (two lane)?
ft ft
nYes nNo nNo standard n Don't know Action: n Leave as is n To be revised
5.3. Are shared driveways allowable?
nYes nNo nNo standard n Don't know Action: n Leave as is n To be revised
5.4. Are alternative materials and designs (i.e., porous pavers, two-track design) allowed?
nYes nNo nNo standard n Don't know Action: n Leave as is n To be revised
PARKING
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6.1. Are parking ratios expressed as both minimum and maximums?
n Yes n No, minimum only n No maximum only n No, Expressed as medians
A ction : n Leave as is
n To be revised
6.2. Are the minimum required # of parking spaces less than: # of spaces
3 spaces perlOOO sq ft for professional office building? nYes nNo n No Standard
4.5 spaces per sq ft for shopping centers? nYes nNo n No Standard
2 spaces per single family home? n Yes n No n No Standard
A ction : n Leave as is
A ction : n Leave as is
A ction : n Leave as is
6.3. Are parking requirements reduced for shared parking arrangements, structured parking, areas near mass transit, and
nYes, all n Not all n Not for any n Don't know Action: n Leave as is
6.4. Are model shared parking agreements provided?
n Yes n No n Shared parking not allowed n Don't know
6.5 Are there special design standards for urban village centers?
n Yes n No n No standard n Don't know
A ction : n Leave as is
A ction : n Leave as is
n To be revised
n To be revised
n To be revised
special districts?
n To be revised
n To be revised
n To be revised
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8. Landscape
dimensions
requirements
7.1. Are minimum
n Yes n No
7.2. Are minimum
n Yes n No
stall dimensions for standard parking space 9 x 18 feet or less?
nNo standard n Don't know Action: n Leave as
driving aisle widths for standard two-way traffic 22 feet or less?
nNo standard n Don't know Action: n Leave as
7.3. Are smaller compact car stalls required for at least 30% of total parking spaces?
n Yes n No
8.1. Does a portion
n Yes n No
ft
is n To be revised
ft
is n To be revised
%
nNo standard n Don't know Action: n Leave as is n To be revised
of impervious parking area require shading with mature tree canopy cover?
nNo standard n Don't know Action: n Leave as is n To be revised
8.2. Is the minimum landscaping requirement at least 20% of the total parking area?
nYes nNo nNo standard n Don't know Action: n Leave as
%
is n To be revised
*See these checklists for a more extensive set of evaluation questions that include additional site design factors.
SUMMARY OF STANDARDS TO REVISE
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