Low Impact Development Manual for Michigan - OSEH - University ...

also providing an infiltration and recharge area for the
impervious roadway stormwater (Maryland Stormwater
Design Manual, 2000).
Street length
Numerous factors influence street length, including
clustering techniques. As with street width, street length
greatly impacts the overall imperviousness of a developed
site. While no one prescriptive technique exists
for reducing street length, alternative street layouts
should be investigated for options to minimize impervious
cover. Successful clustering design consistently has
shown to reduce required street lengths, holding development
programs constant (i.e., 100 homes successfully
clustered on a 100-acre property results in a significant
reduction in street length and total imperviousness than
100 homes conventionally gridded in large-lot development
format).
Cul-de-sacs
The use of cul-de-sacs introduces large areas of
imperviousness into residential developments. Some
communities require the cul-de-sac radius to be as large
as 50 to 60 feet. Simply reducing the radius from 40 feet
to 30 feet can reduce the imperviousness by 50 percent
(Schueler, 1995).
When cul-de-sacs are necessary, three primary alternatives
can reduce their imperviousness; reduce the
required radius, incorporate a landscaped island into
the center of the cul-de-sac, or create a T-shaped (or
hammerhead) turnaround (Figure 6.5).
To reduce the radius, many jurisdictions have identified
required turnaround radii (Table 6.4).
Table 6.4
Cul-de-sac turning radii
Source Radius
Portland, Oregon Office of Transportation 35 ft (with fire dept. approval)
Buck County, Pennsylvania Planning Commission 38 ft (outside turning radius)
Fairfax County, Virginia Fire and Rescue 45 ft
Baltimore County, Maryland Fire Department 35 ft (with fire dept. approval)
Montgomery County, Maryland Fire Department 45 ft
Prince George’s County, Maryland Fire Department 43 ft
Source: Adapted from Center for Watershed Protection 1998
A landscaped island in the center of a cul-de-sac can
provide the necessary turning radius, minimizing impervious
cover. This island can be designed as a depression
to accept stormwater runoff from the surrounding pavement,
thus furthering infiltration. A flat apron curb will
stabilize roadway pavement and allow for runoff to flow
into the cul-de-sac’s open center.
A T-shaped turnaround reduces impervious surface even
further – yielding a paved area less than half that of a
30-foot radius turnaround. Since vehicles need to make
a three-point turn to drive out, T-shaped turnarounds are
most appropriate on streets with 10 or fewer homes.
Figure 6.5
Five cul-de-sac options
Source: Center for Watershed Protection, 1998.
LID Manual for Michigan – Chapter 6 Page 111