| Land Use and Growth Management |
| Highway 99W Trip Budget Model |
| By Lawrence M. Conrad, Portland, Oregon 1-503-417-1362, conradl@pbworld.com
|
| What can be done to improve
travel conditions in an area when funding for major infrastructure
is not available and no alternative routes exist? PB's trip budget
model is helping local agencies find the answers to that question
that are best for their communities. |
|
Small towns in rural areas often face similar situations-their
primary road functions as both Main Street and state highway, and
the competing needs are rarely complementary. Local officials would
prefer to develop land parcels along the corridor to serve local economic
development needs, thus creating additional demands on the roadway,
while the state has a strong interest in maintaining high service
levels.
This was the situation facing the City of Sherwood and the Oregon
Department of Transportation (ODOT) along Highway 99W when ODOT directed
PB to create a simple tool that it and the city could use to track
the impact land use approvals on Highway 99W. The model we developed
generates estimates of the number of trips that would result from
land use development proposals based on the characteristics of the
development and the capacity of the highway.
Figure 1: Six Segments of Highway 99 Analysis |
The Issues and Constraints
Sherwood is a small town located just outside the Portland metropolitan
area, where many of Sherwood's residents work. Many of these residents
commute on Highway 99W, which functions as Sherwood's most important
arterial. A majority of new commercial development is located on or
very near the highway and a substantial supply of vacant buildable
land still exists in Sherwood, so as this land is developed there
will be even greater use of the highway. Recently, the town required
that a developer install a new traffic signal on Highway 99W to make
it easier for residents living in new developments in the southern
part of Sherwood to travel to existing commercial areas by way of
the highway.
From ODOT's perspective, Highway 99W provides the main access between
I-5 and a number of towns to the south and west of Portland. Traffic
volumes on this roadway have grown steadily in recent years. Highway
99W is also one of the three corridors that link the Portland area
to the Oregon coast. Weekend recreation traffic produces large traffic
volumes, but the primary congestion problems on the Portland/Sherwood
stretch of Highway 99W occurs during the weekday PM peak hour commute.
The issues we examined included the:
- Absence of alternative routes, which compels
Sherwood residents to use Highway 99 for local trips
- Increasing through traffic on the highway
- New land use approvals with direct and indirect
access onto the highway
- Lack of local, county or statefunding to make major infrastructure
improvements.
Short-term infrastructure solutions were not available, so we suggested
developing a planning tool that would allow both Sherwood and ODOT
to assess the impact of proposed development in order to allow better
management of the existing facilities. Both parties agreed to this
approach.
Developing the Trip Budget
Model
The trip budget model can be used to generate scenarios that compare
traffic impacts of different types of land uses along a corridor,
such as the effect of altering the mix of office, retail, and residential
mixes. The model is intended to function for up to six years, after
which cumulative changes in conditions will require that new trip
estimates be prepared.
The model uses information from five sources:
- The regional transportation model, which
enabled us to derive evening peak-hour trip estimates for the
current year
- The regional geographic information system
(GIS), which provided information at the parcel level on the supply
of vacant land in Sherwood
- The Institute of Transportation Engineers
(ITE) Trip Generation Handbook (5th edition, 1991), which
provided trip generation rate estimates for new developments
- The city's planning department, which provided
a list of pending and approved land use actions by parcel
- Traffic engineering reports developed for
land use proposals, which included the directional allocations
of trips along the highway.
Trips predicted for previously approved but not yet constructed land
developments were added to the estimates of current traffic, along
with the forecasted growth in the number of through trips. Subtracting
the total estimated number of trips from the total capacity produced
the remaining trip budget. Based on the location of the four major
intersections, Highway 99 was divided into six segments (Figure 1
on the previous page) and capacity was estimated for each segment.
We used GIS to identify all vacant, developable parcels within 1220
m (4,000 feet) of the highway. These lots were classified by zone
and grouped into eleven trip generation areas, each of which feed
traffic onto the highway at one of the major intersections. The ITE
Trip Generation Handbook rates were used to estimate the trips
produced by proposed land uses. The only trips accounted for by the
model were those leaving a trip generation area and using the south
bound lanes during the evening commute.
In the future, whenever a new land use proposal is considered, it
can be entered into the model and the resulting changes in the trip
budget summary can be determined. This ability allows for the rapid
creation of development scenarios in the city.
Although running the model is fairly straightforward, there is quite
a bit of subtlety possible in its application. For example, the capacity
information for a given model run is dependent upon a number of assumptions
that can be changed, leading, in turn, to substantially different
outcomes. The creation of a new signalized intersection at the last
major unsignalized intersection on Highway 99W, for instance, would
require adjustment to several of the model's assumptions.
Model Will Assist Policy Makers
Using the model for information about the amount of capacity available
on a highway implies that policies will need to address how capacity
will be allocated over time. There are several possibilities listed
below, many of which can be used in combination.
- Allocate everything on a first-come/first-served
basis until capacity is used up
- Allocate a predetermined percentage each
year
- Adopt trip caps by parcel
- Adopt a permitting system that scores projects
based on their traffic impacts.
At some point, the full capacity of a highway to carry traffic will
be reached. That point is not entirely fixed, however, and numerous
actions can be taken by governing agencies to stretch the capacity
of a highway, including the following:
- Change land use designations. Retail land
uses tend to generate the highest number of trips while residential
and non-retail commercial uses generate far less. Zoning alterations
can therefore be an effective way of decreasing traffic impacts
of development. This strategy (as well the one that follows) offers
both ease of implementation and an effective approach to decreasing
the traffic impacts of development.
- Identify less trip-intensive zoning along
the highway and rezone the land directly adjacent to the highway.
- Implement impact fees to pay for capacity
improvements. (Clarification as to whether impact fees can be
gathered for a state facility would be needed first, however.)
- Develop site planning standards to minimize
impacts to the highway.
- Adopt the lowest possible level of service
(LOS) standard. (Metro, the elected regional government that is
responsible for transportation and land use planning for three
Oregon counties and twenty-four cities, including Sherwood, has
proposed a 2-hour LOS is E/F)
- Buy the direct access rights to Highway 99W
for specific parcels along the corridor to minimize new trips
directly entering the highway.
- Construct local street projects to serve local traffic and thus
take local trips off the highway.
Many of these actions were previously discussed by ODOT, the city
and PB, but it was decided to develop the model to track the changes
over the next few years.
The model is a simple planning tool for tracking the impact of land
use approvals along a highway. It can be adapted for use in a variety
of similar situations where a state highway functions as an urban
arterial. |
|
| Larry Conrad is a Lead Planner with extensive
experience in planning and research in the public and private sectors.
He was a principal author of the report entitled "Full Cost of
Alternative Land Use Patterns" (Federal Highway Administration),
for which he developed a prototype model (SCALDS) to estimate the
full cost of alternative land use patterns at the metropolitan planning
organization (MPO) level. His experience includes land use planning
at the city, county, and regional levels, facility planning, public
involvement coordination, economic and fiscal analysis, and the creation
and operation of a geographic information system (GIS) for a city. |
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