Land Use and Traffic Congestion - SSTI

LandUse
andTrafficCongestion
FinalReport618
March2012
Preparedby:
J.RichardKuzmyak
TransportationConsultant,LLC
SilverSpring,Maryland
InAssociationwith:
CaliperCorporation
Newton,Massachusetts
and
PolyTechCorporation
Surprise,Arizona
Preparedfor:
ArizonaDepartmentofTransportation
incooperationwith
U.S.DepartmentofTransportation
FederalHighwayAdministration
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EXECUTIVESUMMARY
The purpose of this project was to analyze and interpret the relationship between higherdensity
development and traffic congestion. Governments have expressed increased
interest in the possible benefits of compact, mixed land use—referred to in many circles
as smart growth—to reduce auto dependency and use. If true, this finding could be of
significance in planning solutions to a host of transportation system investment,
performance, and impact issues.
Before considering any type of formal policy position in relation to land use, the Arizona
Department of Transportation (ADOT) is expanding its understanding of the relationships
between land use development patterns and transportation. Among ADOT’s key
questions are:
Does higher-density development reduce auto use, to what extent, and in response
to what factors?
Does higher-density development also generate higher levels of traffic congestion
simply due to the higher concentration of activity?
Do Arizonans know about smart growth, and what are their perceptions of its
impacts and desirability?
The research study that is summarized in this report was commissioned to address these
specific issues. It involved a national-scale review of research and evidence on
transportation and land use relationships; detailed local analysis of these relationships
using data from metropolitan Phoenix; and a survey of officials in Arizona’s metropolitan
areas about their perceptions of land use/transportation, how higher-density development
is viewed, and whether there would be receptiveness for compact, mixed-use approaches
regionally and in their own area.
The findings of this study confirm the benefits of better land use. In its assessment of a
prodigious volume of research on this topic, the project’s literature review was able to
highlight the following findings:
Density and Vehicle Miles Traveled Using residential density as a primary
indicator of concentrated land use, a variety of studies have shown that
households in higher-density (i.e., more urban) settings tend to own fewer
vehicles, drive less, walk and take transit more often, and generate one-half to
one-third of the daily vehicle miles traveled (VMT) of their suburban
counterparts.
Beyond Density: Research has found that the effects of land use on travel
behavior are rooted in factors beyond simple density. Also important are related
factors such as mix of uses, auto- vs. pedestrian-oriented design, and regional
accessibility enhanced by multiple travel choices (especially transit). These
characteristics of density, diversity, design, and destinations are commonly
referred to as the 4Ds.
Travel Purpose: Work travel, which is associated with peak period congestion,
generally garners most of the attention in transportation planning and policy
deliberations. Indeed, where compact land use is focused around high-quality
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egional transit—at both the origin and destination of a journey—commuters will
use transit in large numbers because of its convenience. However, the travel
market that may be most influenced by compact mixed-land use is nonwork
travel, which accounts for as much as 80 percent of routine household travel and
has been the fastest-growing segment since the 1980s. This relationship/trend can
be directly linked to land use, recognizing that in conventional suburban areas
almost all household needs—shopping, transporting children, personal business,
social, and recreation—require private vehicle travel. Areas where residents live
in older, mixed-use communities with nearby services and restaurants show a
much greater concentration of travel to local destinations—including walking,
biking, or short car trips—despite a daily commute that may well be a long
distance solo-driver trip.
Market Forces: Critics of smart growth approaches to land use maintain that it is
a planner’s notion that does not reflect market realities. However, real estate
industry experts assert that the reason more compact, mixed-use development has
not occurred has to do with restrictive local zoning codes or traffic level of service
standards, and not because of market demand, which is gauged as twice as high as
current build rates. This is borne out by visual preference surveys that show a
general preference for older (pre-World War II) suburban development patterns,
which are more compact and walkable, and foster more social interaction between
residents.
To ascertain the validity of these research findings in the Arizona environment,
researchers performed a number of detailed studies using local data and both existing
planning tools as well as some new ones developed specially for analysis of the role of
land use. These analyses were focused on the Phoenix metropolitan area and were
performed using data, modeling tools, and staff support of the Maricopa Association of
Governments (MAG).
Conventional four-step transportation planning models are unable to account for
important differences in land use as represented by the 4Ds. The influence of land use is
most relevant at the level of the individual traveler and what they can walk to within ¼ to
½ mile at either origin or destination. Four-step models operate at a traffic analysis zone
(TAZ) level of aggregation, which is generally much too coarse for discerning land use
differences. For this reason, this genre of models also does not deal directly with
nonmotorized trips such as biking or walking, which are a critical element in compact
mixed use designs.
Using data from MAG’s 2001 regional household travel survey supplemented with
information from its travel model and geographic information systems (GIS) databases,
researchers developed a set of regression models to quantify the relationships between
travel behavior, traveler demographic characteristics, and measures of the 4Ds. These
models show the effect of the 4Ds of land use on both household vehicle ownership and
on household VMT. Residential density, land use mix, walk opportunities, and regional
transit accessibility to jobs were the variables used to represent the 4Ds. Negative signs
on the coefficients for these variables indicate that as each of the land use variables
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increases, vehicle ownership and VMT rates decline proportionately. Vehicle ownership
is an important determinant of travel in the VMT models, so when it is reduced in
relation to better land use, its effect is compounded by also acting to reduce VMT.
The MAG region was divided into 17 jurisdictional areas and the household travel survey
database used to explore differences in travel in relation to these key land use factors.
Higher-density and more mixed-use areas such as South Scottsdale, Tempe, and East
Phoenix were found to behave significantly differently from lower- density/less mixeduse
areas like Glendale, Gilbert, and North Scottsdale. Residential density for the more
compact areas ranged from 6.14 to 6.94 households per acre vs. 2.86 to 3.61 households
for the lower-density group. These higher-density areas also had better mix (0.53 vs. 0.30
value on a 0 to 1.0 entropy index scale); more retail and service opportunities within
walking distance (42.4 vs. 15.4); and considerably more jobs accessible by transit (59,000
vs. 27,000). The implications of these differences may be seen in various travel measures,
including:
Vehicle Ownership: 1.55 vs. 1.92.
Average Trip Lengths: 7.4 vs. 10.7 miles for home-based work trips; 2.7 vs. 4.3
for home-based shopping trips; 4.4 vs. 5.2 for home-based other trips; and 4.6 vs.
5.3 for nonhome-based trips.
Per Capita VMT: 10.5 miles per day vs. 15.4 miles per day.
The 4Ds models were subsequently used to investigate the potential impact of improved
land use characteristics in each of the 17 areas. To do this, average residential densities
were raised to 10 households per acre (vs. on the order of two to four in most places),
land use mix was brought to the ideal entropy index value of 1.0, the number of walk
opportunities was increased to 100 in all places, and regional transit accessibility was
raised to somewhere between the current minimum and maximum for the respective area.
This resulted in estimates of VMT reduction of 20 percent to 45 percent, with an average
overall of 25 percent.
Having reasonably demonstrated that areas in Phoenix with higher density generated less
vehicle travel per capita than lower-density areas, the second hypothesis investigated was
whether a higher concentration of activity would also lead to localized traffic congestion.
A sample of four urban corridors was selected for detailed study—also in the Phoenix
area—based on information from local and regional officials that these areas were
perceived to have major traffic congestion issues: Scottsdale Road between Thomas Road
and Chaparral Road in the older, southern part of Scottsdale; Central Avenue north of
downtown Phoenix, between McDowell Road and Camelback Road; the Mill Avenue
and Apache Boulevard corridor through Tempe; and West Bell Road in the northwest
part of the region, connecting the central valley with the newer communities of Peoria,
Glendale, and Surprise.
The objective was to examine the interplay between the intense development patterns in
these areas and the condition of traffic on the street and road network. Researchers
performed the following assessments:
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Density: Three of the areas—south Scottsdale, Central Avenue, and Tempe—
exhibited some of the highest development densities in the region while Bell Road
served an area of intense activity spread over a large area of moderate to low
density.
Composition: The Scottsdale, Central Avenue, and Tempe areas also had a high
level of mix in their land uses, while Bell Road was heavily residential. The
overall jobs-to-housing ratio in the Bell Road area was only 0.49 compared to
1.42 in Scottsdale, 2.30 in Tempe, and 5.60 in the Central Avenue corridor. Retail
jobs per household (a measure of access to local services) was not quite as
skewed, but Bell Road’s ratio of 0.31 was still only about half of the 0.56 to 0.65
level found in the other three areas.
Road Network: Each area is served by the one mile arterial “super grid,” with no
area having a freeway closer than two miles from its center. However, a major
distinction occurs in the secondary road system, with Central Avenue and
Scottsdale having a rich network of secondary streets on one-eighth mile spacing.
Tempe’s secondary grid is not quite as fine but is still much better than the Bell
Road corridor, which has little secondary road system beyond subdivision
networks.
Transit: Central Avenue and Tempe are well-served by the regional bus system
and are also connected by the region’s inaugural light rail line (not operational at
the time of the analysis). Scottsdale is moderately served by transit, while Bell
Road has only park-and-ride bus service. Transit accounts for about 6 percent of
all internal trips in Scottsdale, and between 3 percent and 6 percent of external
trips. In the Central Avenue corridor, about 8 percent of internal trips and about 7
percent of external trips are made by transit, while in Tempe about 3 percent of
internal trips and 5 percent to 10 percent of external trips are by transit. In
contrast, less than 1 percent of all trips in the Bell Road corridor involve transit.
Traffic Congestion: Interestingly, traffic congestion levels were much lower in
the Scottsdale Road and Central Avenue corridors than in the Bell Road corridor.
Volume-to-capacity (V/C) ratios in both Scottsdale and Central Avenue were in
the 0.8 to 0.9 range in the PM peak period compared to 1.6 to almost 2.0 along
Bell Road. Tempe fell predictably in between given its less-articulated secondary
road network, with V/C ratios in the neighborhood of 1.0. Tempe also employs
traffic-calming strategies on its secondary road network to discourage cut-through
traffic, which pushes traffic onto major arterials.
Through Traffic: Traffic volumes in each of the four areas are affected by
through travel (no trip end within the defined area). Central Avenue, Bell Road,
and Apache Boulevard in Tempe all had rates of through travel that accounted for
about half of peak period traffic volumes. Without this through traffic movement,
Central Avenue and Tempe would be relatively uncongested, though Bell Road
would still be congested from its internal volume. Scottsdale’s rate of through
traffic on the measured links was much less—about 22 percent to 28 percent—
probably due to the design of the local grid, which encourages through travelers to
use peripheral streets.
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These findings tended to corroborate responses elicited from participants in the project’s
survey of officials. For this survey, which was conducted early in the project, researchers
distributed 423 questionnaires and received 134 responses from a diverse list of elected
officials, planning and zoning officials, transportation planners, and members of other
relevant disciplines in the Phoenix, Tucson, and Flagstaff metropolitan areas. Some of the
key discoveries in this investigation are given below:
Traffic Congestion Concerns: While important, traffic congestion was rated as
less a factor in project review than were issues of compatibility with adopted
plans and impact on surrounding neighborhoods and businesses.
Familiarity: Most officials were familiar with mixed-use concepts, had been
involved in the review of these concepts, and had even encouraged submission of
such projects.
Transportation Impacts: The overwhelming majority of officials responding
believed that compact, mixed-use development would increase transit use and
nonmotorized travel, though only about one-third felt unequivocally that it would
lead to less traffic congestion. (Most were unsure.)
Desirability: The great majority believed that the region would benefit from more
mixed-use centers and corridors, focusing employment in centers and corridors,
and building more mixed-use communities. About 80 percent believed that their
own community would support compact, mixed-use development.
Residential/retail and office/retail mixed use were the most highly rated
combinations.
These findings suggest an opportunity to advance the dialogue on and support for
compact, mixed-use development in Arizona’s metropolitan areas. Among the initiatives
that might be considered are the following:
Education: There is a need to better inform the public, the business community,
and officials about the nature and benefits of compact, mixed use. Themes
developed in this project can serve as educational messages.
Better Analysis Tools: Local planners and planning commissions are still using
traditional traffic engineering approaches to assess the impact of development
projects. By looking only at traffic congestion levels on adjacent links, ignoring
through travel, and failing to account for the efficiencies of mixed-use
development on lower vehicle trip rates and VMT, progressive projects are likely
to be rejected or unreasonably downsized. The metropolitan planning
organizations should take steps to add 4D enhancements to their existing tools.
Visioning and Plan Overhauls: Existing long-range or comprehensive plans
may be silent or devoid of a position on compact, mixed-use development.
Regional or local targeted visioning exercises can raise visibility and
understanding of the issues, leading to greater acceptance and support in updated
plans.
Incentives: Adoption of compact, mixed-use development approaches can be
encouraged in various ways. Grant monies and/or technical assistance can be
offered to support studies or demonstration projects. Several states prioritize state
program or grant funding based on demonstrated steps by a jurisdiction to
embrace and incorporate key elements in their plans, codes, or procedures.
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Supportive Infrastructure: A key incentive in its own right, local land use
choices can be influenced by the manner in which transportation resources are
distributed. Priorities can be placed on investments that will most contribute to
concentrated land use policies such as transit investments, local street and
sidewalk infrastructure, or rehab/upgrade of facilities in older developed areas.
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