From: Residential Car Washing
Note
The first step in estimating the number residential car washes draining to the MS4 was to multiply the number of Federal Way registered vehicles (62,000) by 38% (ICA 2005). This provides a figure for Federal Way driveway car washes (23,560). Next, a series of theoretical car washing frequencies was derived by multiplying the driveway car wash figure by each of the listed percentages (i.e., once a week, two or three times per month, etc.) found in the Lake Sammamish Hardwick study data (see table). The six individual car wash frequency categories were then converted into annual number of washes (i.e., once per week multiplied by 52) and combined into a total (444,224). This total represents the estimated annual number of residential car washes in Federal Way draining to the MS4. Each pollutant concentration was then converted into mass using this estimated total number.
| Car Wash Frequency |
Percentage of home car washers |
Number of Federal Way driveway car washes |
Annual number of Federal Way driveway car washes |
| Once per week |
11% |
2,592 |
134,763 |
| 2 or 3 times per month |
27% |
6,361 |
190,836 |
| Once per month |
32% |
7,539 |
90,470 |
| Every other month |
14% |
3,298 |
19,790 |
| 2 or 3 times per year |
13% |
3,063 |
7,657 |
| Once per year |
3% |
707 |
707 |
| Total Number of Annual Driveway Car Washes Draining to MS4 |
|
|
444,224 |
September 2009
From: Residential Car Washing
Note
The first step in estimating the number residential car washes draining to the MS4 was to multiply the number of Federal Way registered vehicles (62,000) by 38% (ICA 2005). This provides a figure for Federal Way driveway car washes (23,560). Next, a series of theoretical car washing frequencies was derived by multiplying the driveway car wash figure by each of the listed percentages (i.e., once a week, two or three times per month, etc.) found in the Lake Sammamish Hardwick study data (see table). The six individual car wash frequency categories were then converted into annual number of washes (i.e., once per week multiplied by 52) and combined into a total (444,224). This total represents the estimated annual number of residential car washes in Federal Way draining to the MS4. Each pollutant concentration was then converted into mass using this estimated total number.
| Car Wash Frequency |
Percentage of home car washers |
Number of Federal Way driveway car washes |
Annual number of Federal Way driveway car washes |
| Once per week |
11% |
2,592 |
134,763 |
| 2 or 3 times per month |
27% |
6,361 |
190,836 |
| Once per month |
32% |
7,539 |
90,470 |
| Every other month |
14% |
3,298 |
19,790 |
| 2 or 3 times per year |
13% |
3,063 |
7,657 |
| Once per year |
3% |
707 |
707 |
| Total Number of Annual Driveway Car Washes Draining to MS4 |
|
|
444,224 |