More Bad Reports - Street Sweeping. Part 3

We began our analysis of the City’s report on street sweeping by looking at the fact that the City’ hypothesis (water quality is impacted by urban runoff) and their theory (blocked streets result in less trash pickup) had little if any basis in fact. Then we looked at the City’s study itself. Today we’ll discuss some of the flaws.

NOT THE RIGHT DEPENDENT VARIABLE

No Measure of Debris

If the main goal of the investigation is to reduce trash and debris, one would think that there would be a measure of trash and debris, yet at no point in the study is this ever considered, even though many municipalities have looked into this. Instead, the report focuses on an indirect index that may or may not be related to trash and debris getting into the sewer system – street blockage by cars.

No Measure of Localized Flooding

One of the thoughts about street sweeping is that it may prevent localized flooding by keeping the drains clear. Yet at no time has the City tried to examine those areas that suffer from localized flooding to determine whether blockage is a problem there. As far as I know, the 5 streets where they found high blockage have never suffered from localized flooding. OTOH, major flooding (causing tens of thousands of dollars of damage) has happened on Ridge Route between Overlake and Muirlands, yet this street was not selected.

NOT THE RIGHT MEASUREMENT

If you put aside the fact that the empirical evidence shows that street sweeping is not going to impact water quality, and you put aside the fact that the City did not choose to measure the amount of trash and debris picked up, we can focus instead of the City’s measuring tool – street blockage.

Debris vs. Blockage

The City seeks to measure effective removal of debris by trying to reduce blockage by cars. There are several fundamental flaws in this type of thinking. First, not all streets in all neighborhoods have a lot of debris, so that a relatively clean street with heavy blockage will not produce as much debris as a relatively dirty street with little blockage. We learn from the report that some streets have as high as 50% or more blockage, but other streets have blockage less than 10%. Blockage, per se, is not the issue, but rather debris. The city has no measure of the amount of trash and debris on these streets, which would be the main determinant, not blockage. So they are going ahead and restricting parking on streets where they have no idea that the actual amount of trash and debris is excessive.

Street Structure

Even if one were to conclude that blockage was a major variable, and put aside the real dependent variable (i.e., debris), the effectiveness of street sweeping is a function of the interaction between the blockage and the shape of the street. For example, I live on a street that is vaguely U-shaped. Most of the debris collects at the low point in the street. If this small section is blocked by a single car, the street sweeper will miss almost all of the debris. But if the ends of the street are occupied by a dozen cars, and the low point is free, the street sweeper will collect most of the debris from my street. The amount of debris collected is not a function of blockage, but the blockage at the critical point(s). The City's analysis completely ignores this crucial variable.

Even the most superficial study of local streets will show that a large number of streets are not on a level plane. In addition to my U-shaped street, adjacent streets are on a slope, and on these streets the debris tends to collect at the bottom of the slope. Again, there can be a dozen cars at the top of the slope and a street sweeper will collect most of the debris if it has access to the bottom of the slope. IOW - if a street is sloped, it may not matter that the bulk of the street itself is blocked - what matters is whether or not the bottom of the street, where the trash congregates, can be swedpt. In almost all cases the answer is "Yes" because parking restrictions keep most of these areas free.

(FWIW - A study in San Diego showed that different methods of street sweeping were differentially effective depending on the type of street. For a flat street, Vacuum did twice as good as Mechanical, but on a steep surface, Mechanical outperformed Vacuum by nearly 20%.)

NO BENEFIT FROM EXPERIENCE

The City report notes that street sweeping restrictions were placed on 10 residential streets in 2010, but then fails to provide any information. How successful has this been over time? What are the ticket counts? Do they change over time? What percentage of the streets are still occupied? Did that change over time?

Here we have vital information about the long term success of street sweeping restrictions, but the report fails to provide any information.

These are but a few of the fundamental flaws in the City's research, after you put aside the major flaws in formulating the study in the first place. This type of research has all the sophistication of a first year college student, yet the people who authored this report cumulatively receive more than $500,000 from the City.

Tomorrow we'll conclude.