Informed but opinionated commentary and analysis on urban transportation topics from the Secret Worldwide Transit Cabal. Names have been omitted to protect the guilty.

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A Wendell Cox "MacGuffin" a la Hitchcock?

From the Cabalmaster:

Alfred Hitchcock was famous for "MacGuffins" in his movies. "MacGuffins" are, as defined by this Hitchcock site:

www.labyrinth.net.au/~muffin/subscribing_c.html

"The journal is named after Hitchcock's/ Angus MacPhail's name for a diversionary plot device. (So perhaps our 'MacGuffin' is just a pleasant diversion from the films themselves!) Every Hitchcock buff knows the concept of the MacGuffin. In our first issue, we compared it to T.S. Eliot's definition of 'meaning' in poetry: how it's like the bone thrown by the burglar to distract the watchdog of the mind - while the poem, the burglar, goes about its real business".

From what we in the Transitcabal have seen, this definition of "MacGuffins" also accurately describes much of what Wendell Cox does.

Now whether the following Cox item qualifies as a “MacGuffin” remains to be seen. How this might also be used in future analysis (sic) remains to be seen:

www.demographia.com/db-jhbal1990walk.htm

"WORK TRIP WALKABILITY IN US METROPOLITAN AREAS
Neighborhood Jobs-Housing Balance Index, 1990 (Draft)"

As Cox describes this:

“The Neighborhood Jobs-Housing Balance Index is average variance of jobs and housing (lesser divided by greater) by transportation analysis zone (TAZ) (or other measure used in the metropolitan area) as reported in USDOT Census Transportation Package 1990. This theoretical home-work walkability indicator does not take into consideration how well local residents might match to local jobs.

Perfect balance = 1.000.

Range = 0.000 to 1.000.

Higher score indicates greater balance (greater walkability).
Additional details below."

The measure Cox is reporting is interesting, I suppose, but it is only a very crude indicator of "walkability." At an aggregate level, such a measure doesn't reveal much without more context. How “theoretical walkability” compares to real “walkability” is discussed below.

BTW, "Neighborhood Jobs-Housing Balance" is a different concept than "walkability," in the sense that most analysts have used the latter term. If a TAZ has a balance of jobs on one side but is separated from residential by dead-end streets or a freeway, "walkability" is obviously near nil. There is no way to measure the actual, on-the-ground "walkability" without actually going out and looking, or spending a lot of time with maps, aerial photos, and a GIS, if one is lucky enough to have access to a GIS system.

Regardless of what one may think of the LUTRAQ work that was done in Portland, one of the useful things was the "Pedestrian Environmental Factor" ("PEF") that came out of that study, was the following report:

ntl.bts.gov/DOCS/tped.html

"4. Overview"

"This volume is a supplement to Volume 4 of the LUTRAQ Project, issued in November, 1992. In that report the authors describe several travel model enhancements made to the forecasting system used by Metro in Portland, Oregon. Of greatest relevance here is a description of a new variable, called the "Pedestrian Environmental Factor" (PEF). This measure is a composite of four different attributes of the natural and built environment, which were shown to improve the accuracy of several sub-models in the Portland system. In particular, the pre-mode choice model (walk/bike vs .motorized modes) was enhanced by the introduction of the PEF measure into the set of equations on which the model was calibrated. As developed by the Metro staff in consultation with the LUTRAQ Consultant Team the PEF consists of an assessment of each of 400 zones in the regional travel demand forecasting model network for each of the four following parameters:

- Ease of street crossings
- Sidewalk continuity
- Local street characteristics (grid vs. cul de sac)
- Topography

"To estimate ease of street crossings at the zonal level, staff identified key intersections and evaluated their width, extent of signalization and traffic volumes. For a measure of sidewalk continuity, staff judged the extensiveness of sidewalks on principal arterials served or likely to be served in the future by transit. Secondary attention was paid to the extent of sidewalks on neighborhood collector streets. As a measure of connectivity of street systems, staff estimated the extent of grid street patterns in each zone. They also examined the fineness of the grid (distance between intersections). As a measure of topography staff evaluated the extensiveness of sloping terrain and the steepness of these slopes. Each zone in the model system was scored on a three-point scale for each of the four characteristics named above. A composite score (four-12) was created for each zone, with 12 being the highest possible and four the lowest. Four different staff completed this exercise independently and then compared results in order to enhance the objectivity of the analysis. Some zonal scores were modified on specific parameters to reflect a consensus reached on their characteristics. This simplified Delphi approach resulted in consensus on rankings for the entire network of zones. Table 1 summarizes the distribution of pedestrian environmental factor scores. Figure 2 shows the location of the 400 traffic analysis zones used in the analysis and Figure 3 summarizes the distribution of the PEF scores associated with each of them."

Since it is a direct measurement "on the ground," PEF seems a far more useful measure of "walkability" than how Cox is trying to equate jobs-housing balance with “walkability.” Fortunately PEF is something that can also be measured relatively easily using aerials and GIS, if one wants to take the time and has access to the data. I'll leave this to planners who have daily access to such nice databases. This is a good exercise that all General Plan updates should incorporate.




posted by The Cabalmaster, Master of All Transit-dom 8/22/2002 01:04:00 AM