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Pavement Sweeping | De-Icing | Street Trees and Landscaping
Curbs and Gutters | Road Widths | Parking Lot Size and Surface
Streets and Roads Regulations

Land use decision-makers historically perceived stormwater runoff from roads and highways as primarily an issue of safety – with the focus on directing and draining water off paved surfaces quickly and efficiently to avoid car accidents. Once off the road stormwater was no longer addressed.

Recently planners have begun broadening their focus to include issues of flooding and erosion. But including such problems in planning and regulation matters is only a starting point -- reducing pollutants in stormwater runoff, and their effect on water quality remains the critical issue to address. (1) Furthermore, a recent study found that street widths have a causal relationship to accidents. The study found narrower streets created slower traffic and fewer accidents, while wider streets encouraged speeding and dangerous conditions.

According to the US EPA, our transportation systems, which include roads and parking lots, account for as much as 70 percent of the total impervious surface in urban areas. (2) Research conducted in Wisconsin by Roger Bannerman of the Wisconsin DNR focused on examining the concentration of pollutants from specific categories of surfaces. The study looked at the runoff from 12 different types of surfaces such as driveways, lawns, parking lots, rooftops and streets. Results showed that all land uses are not equal when it comes to generating contaminants in runoff. Streets were clearly identified as the type of surface that creates the highest pollutant load, contributing 54 percent of all runoff volume in residential neighborhoods and 31percent of runoff in commercial neighborhoods (streets and parking lots combined to contribute 80 percent of runoff in commercial neighborhoods). (3)

In addition, there are documented case studies conclusively linking increased watershed imperviousness to negative hydrologic impacts on streams. (1) Research in Wisconsin has shown that relatively low levels of imperviousness (15%) cause stream degradation (2). Over 60 percent of all impervious surfaces are related to automobile transportation, including driveways, roads, parking lots and highways. (3)

Given that our transportation systems (roads, driveways and parking lots) account for the majority of impervious surfaces, and streets and roads in particular cause the most concentrated pollution loads, it is logical that streets and roads be the centerpiece of efforts to control stormwater related pollution. It is in a community's best interest to reduce the width of streets for a number of reasons. The benefits that stem from street width reductions provide communities the opportunity to limit the impact runoff pollutants have on surrounding waterbodies, reduce initial construction and maintenance costs, and slow traffic flow to alleviate safety concerns.

Pavement Sweeping --
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Pavement sweeping is used to remove sediment, debris and other pollutants that have accumulated on paved surfaces. Pavement sweeping is of particular importance before spring snowmelt because it reduces pollutant loads from road salt and sand that would otherwise be carried away in runoff. Innovations in equipment design and technology have improved the performance of pavement sweepers, making them more capable of removing finer sediment particles than the machines used in earlier decades.

Regulation of parking is an important consideration in developing a sweeping program so that the sweepers are able to get at curbs and do an effective job. An associated consideration is the frequency of the scheduled sweeps. Computer modeling from the Pacific Northwest indicated that the optimum frequency appears to be once every week or two. Additional factors in developing programs include coordination of the program with parking regulations, the need for proper sweeper operator training and finally, accurate tracking of 1) which streets have been swept and 2) how often. The frequency and intensity of rainfall for a region along with the conditions of the street are also factors affecting the overall ability of street sweeping programs to reduce non-point pollution. (5)

Arranging for proper disposal of collected “swept” materials may also pose a problem because of possible high levels of lead, copper, zinc and other wastes from automobiles. However, this disposal problem points to the importance of removing the wastes from streets and parking lots before storms wash them into waterways. (6)

Pavement sweeping programs require a significant investment of capital and a yearly operation and maintenance budget. It is important to use newer, more sophisticated equipment – with costs approaching $200,000 because studies of old equipment showed the sweepers were not very effective. These sweepers generally have a useful life of about 4 years, however, proper maintenance of the machines will greatly improve sweeping efficiency. (7)

Sweeping of parking lots at industrial sites can be conducted without use of mechanical sweepers. In this case, sweeping involves using brooms to remove small quantities of dry chemicals and solids from areas that are exposed to rainfall or storm water runoff. Sweeping and disposal of materials is a reasonable inexpensive method of pollution prevention – and it requires no special training or equipment. (2)

Given the results of Wisconsin-based studies demonstrating that streets and parking lots contribute the majority of both the total runoff volume and concentration of pollutants, sweeping programs that provide for periodic removal of trash, litter and particulates from pavement will remove a portion of pollutants and can significantly reduce the stormwater pollutant load contributing to urban runoff.

De-icing -- Back to Top
Given Wisconsin’s harsh winters and seasonal snowfalls, road de-icing, with salt as the primary medium, is a very common community practice. However, the salt and sand used on roads can be large contributors to runoff and ultimately to receiving streams when these materials are carried into the storm drainage system by snowmelt.

Improper storage of the salt may increase contamination of waterways. Even though covering road salts during storage may seem to create an expense, the benefits of coverage are greater than the costs. Benefits include preventing salt from lumping together, which makes loading and application easier. Also, covering storage piles reduces loss from storm water runoff and therefore, the associated potential for contamination. Locating the storage site outside the 100-year floodplain provides further protection.

Proper application techniques will also decrease runoff. Calibration devices for spreaders aid workers in the proper application so that the minimum amount needed to be effective is used. The amount of salt should reflect site-specific characteristics, such as road width and design, traffic concentration and proximity to surface water. (8)

Road salt can contain cyanide, which may be acutely or chronically toxic to aquatic organisms. Alternatives to salt are recommended, including calcium magnesium acetate (CMA), sand and gravel, cinders, formats and agricultural resides.

Street Trees and Landscaping --
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A 1991 study by the United States Forest Service (USFS) reported that tree canopies in a Chicago study reduced urban storm water runoff by four to eight percent by intercepting rainfall, storing it and then allowing the rain evaporate. In the absence of street trees, the rainfall on pavement runs off more quickly. Urban trees are able to significantly slow the highly polluted “initial flush” of a storm. In many cities, this initial flush bypasses any treatment and flows directly into the city’s waterways. (9)

A 1997 report by the nonprofit group "American Forests" attributed a rise in temperatures in downtown Atlanta that was 6-9 degrees more than those of the surrounding countryside to the fact that the city center had lost 65 percent of its tree cover, creating what scientists call and “urban heat island”. (10) In any urban area, high volumes of runoff from hot paved surfaces and rooftops may cause surface water temperatures to rise quickly.

Trees fill a very important role when located between a community and adjacent agricultural lands. When properly planted, trees may serve as a living buffer against contaminated water and air, dust, noise, wind and blowing snow – originating from both rural and urban areas. Trees have the ability to trap sediments and filter chemicals from roads, parking lots and lawns before they reach the natural watercourse. (11)

Other landscaping practices such as using vegetated areas to buffer or “break up” the asphalt may be used. Preliminary results from the first year of a 2-year study in Florida show that retaining a natural vegetated “swale” or marshy lowland near a parking lot reduced the average runoff amount by 30 percent. (16) Landscaping in parking areas and road medians is frequently placed in a raised bed surrounded by curbing. However, studies show landscaped areas perform their infiltration roles much better if they are planted below the level of the surface where they can truly capture runoff. (17)

Curbs & Gutters -- Back to Top
Curbs and gutters are designed to quickly move stormwater runoff from the street to a storm sewer. Because they’re designed to only move water, they provide for little to no removal of pollutants and can store pollutants until they’re washed away in the next storm.

The U.S. EPA encourages the revision of road and drainage zoning regulations to allow for the use of engineered grass swales as an alternative to curbs and gutters – in certain situations. These situations include low and medium-density residential zones where underlying soils are permeable, the slope is within a 1- 5 percent range and the water table is more than 2 feet of the proposed bottom of the swale. (13) However, the South East Wisconsin Regional Planning Commission (SEWRPC) does not promote the use of grass swales in medium density housing developments because of issues related to their long-term maintenance.

Road Widths -- Back to Top
A concept of road hierarchies has been long established that relates road width to intensity of use, implying that a wide width is needed for roads with heavy use. However, a study by Southworth and Ben-Joseph (1995) found that local planners and commissioners are no longer using this road hierarchy. As a result, roads are often much wider than needed.

Environmental Protection Agency recommends reducing road surface. EPA maintains that road widths in many areas can and have been reduced and still handle traffic and emergency response vehicles in an adequate manner. In addition to lessened stormwater runoff, reducing road width provides other benefits such as construction and maintenance savings, less required clearing and grading during initial construction, traffic calming, and more livable, walkable neighborhoods through increased pedestrian friendliness. Concepts such as traffic calming and livable, walkable neighborhoods are important considerations when planning development. Several real estate studies concluded that low traffic volume, slow traffic speeds and minimal noise were preferred elements in home selections. (14)

Traditional subdivision zoning regulated rooftops density but not roads. However, roads, driveways and parking lots (transport systems) often exceed rooftops as a percentage of the total impervious surface in these subdivisions. Because the zoning measures almost always fail to include roads and driveways in its measure of imperviousness, the calculations leads to a practice that is at odd with its goals. In fact, when compared to traditional neighborhoods, large lot subdivision zoning creates more impervious surface and impacts water resources more negatively than traditional neighborhoods. (13) Some of the principles of conservation development may be useful in reducing pavement. Conservation design embodies a plan that maps the traffic pattern, road layout and preliminary lot layout with the goal of reducing pavement. By planning for narrow road sections and limiting the combined traffic distribution network of roads, sidewalks, driveways and parking lots, the total impervious area for the planned segment is reduced.

An important consideration in determining minimum road width is the ability of emergency service vehicles to quickly reach a fire or medical emergency. Narrower street widths can accommodate emergency vehicles by constructing wider entryways to properties and by restricting parking to only one side of the road. The turning radius provided for in an intersection must be also designed to accommodate these vehicles.

The Center for Watershed Protection developed principles for reducing impervious surfaces.

Their recommendations for residential streets include:

1) Design residential streets for minimum required pavement width needed to support travel lanes; on-street parking and emergency, maintenance and service vehicle access. These widths should be based on traffic volume.
2) Reduce the total length of residential streets by examining alternative street layouts to determine the best option for increasing the number of homes per unit length.
3) Wherever possible, residential street right-of way widths should reflect the minimum required to accommodate the travel-way, sidewalk and vegetated open channels. Utilities and storm drains should be located within the pavement section of the right-of-way wherever feasible.
4) Minimize the number of residential street cul-de-sacs and incorporate landscaped areas to reduce their impervious cover. The radius of cul-de-sacs should be minimum required to accommodate emergency and maintenance vehicles. Alternative turnarounds should be considered.
5) Where density, topography, soils and slope permit, vegetated open channels should be used in the street right-of-way to convey and treat stormwater runoff. (15)

Parking Lot Size and Surface -- Back to Top
Research has shown that in most instances an oversupply of parking exists– resulting in empty parking lots and an unnecessary excess of impervious surfaces. This oversupply is often developed because a retailer, commercial development or zoning ordinance attempts to provide enough parking to satisfy a peak demand that rarely occurs – such as shopping the day after Thanksgiving. Multiple studies have found that standard ratios (4 spaces per 1000 feet of commercial space) are almost double the amount of parking that is actually needed. (11) If the parking supply is adjusted to meet the typical demands rather than a peak period demand, land will be conserved and a much smaller impervious surface created.

Methods to counteract this oversupply include:
--- Reducing the recommended allocation of parking spaces in zoning codes;
(See sample)
--- Expressing the allocation of parking spaces as a maximum rather than a minimum;
--- Allowing on-street parking spaces adjacent to a development to count toward minimum parking requirements.
--- Encouraging shared parking, joined parking or coordinating parking facilities.

Shared parking facilities take advantage of complementary land uses located near each other which have differing times for peak demands in parking. Examples include theaters, restaurant or hotels located near a commercial shopping district.

Studies conducted in Olympia Washington demonstrated that parking reductions of 100 percent (of the smaller lot) can be achieved in complimentary daytime and nighttime land uses share parking and 50 percent reductions (of the smaller lot) are achievable with land uses overlap some hours.

In addition, some cities allow developers to pay a fee in lieu of providing parking spaces, which the community then uses to finance shared public parking spaces developers would have provided. (12)

In some instances where parking lot size has been reduced, communities require sufficient open space be set aside on the site to accommodate the parking spaces that would have been required under the original, larger ratios.

Results of the Olympia, Washington Cooperative Parking Study provide incentives for participating in cooperative parking include:

--- Reduced construction, landscaping and maintenance costs.
--- Decreases in required parking, ranging from 5 to 25% -- if developers agree to share parking.
--- An increased ability for developers to complete projects that would otherwise have been denied due to parking deficits.

Barriers include:
--- Inflexible regulations or rigid regulatory language that limited locations for cooperative parking to those that share a property line
--- Lack of model cooperative parking agreements.
--- Perceptions that large parking lots are a necessity.
--- Lack of any promotion of such arrangement. (13)

Additional methods to decrease the impact of parking lots’ impervious surface include:

--- Designating a portion of the allocated parking space as “overflow.” This is parking that is generally needed only during peak time.

Overflow parking, which would have low usage should be surfaced as a permeable surface using materials such as:

--- Open frame work concrete pavers filled with sand or gravel;
--- Turf reinforced with plastic rings. (14)

The use of pavers must be limited for three reasons. 1) Pavers infiltrate water. By restricting them to low use areas, there is better assurance that only clean water will infiltrate and thereby groundwater contamination will be avoided. 2) pavers are much more costly than asphalt 3) Most pavers do not meet the Americans with Disabilities Act (ADA) standards and thus pose a liability problem for local governments. These systems can become clogged, which is the reason they are primarily recommended for overflow parking. However, studies suggest that permeable surfaces can be used successfully, especially if they are installed properly (that means the system needs to be backfilled with clean sand or better yet – clean gravel), and maintained through semi-annual “vacuum cleaning.” However, the studies conducted in 1999 also concluded that several more years of testing are necessary to provide a definitive answer on clogging. It should be also be noted that some of the testing on the efficiency was conducted near Seattle, Washington – in a climate that experiences winter, albeit not as harsh as that of Wisconsin. (15)

Wisconsin Street and Road Regulations --
To read more about Wisconsin regulations for Streets and Roads, link here

Source for text in oval:
First Oval: DNR, UW-Extension Publication #GWQ016, "Cleaning Up Stormwater Runoff"
Second Oval: DNR, UW-Extension Publication #GWQ020, "Polluted Urban Runoff--A Source of Concern"
Third Oval: Stormwater Center, Fact Sheet, "Better Site Design! Narrower Streets"

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