Case Study:

Albany, New York

Conclusions

Limitations

The CDTC model shares a number of limitations that are common to all land use models, to varying degrees. For example:

• The model is capable of explicitly testing only a very limited number of policies; for example, property tax rates. Most of the factors that make a zone attractive to development, such as schools or parks, are not considered, except through an overall "zonal constant" for each zone. Without further knowledge, it must be assumed that these attractiveness factors remain constant in the future. In addition, the model is calibrated to historical patterns, but there is no guarantee that future development will follow similar patterns.
• There is no disaggregation of households or employment by type; all households or employment are expected to respond similarly to accessibility changes. This is a limitation common to travel demand as well as land use models. A refinement of the model could include disaggregation by income group (for example) if sufficient data were available to estimate differences in residential development factors by income group.
• Only peak-hour accessibility is considered. The importance of off-peak versus peak-hour accessibility, as well as any time-of-day shifts in travel behavior resulting from congestion, are not considered.

In addition, the approach described here has some further limitations that should be noted:

• The model assumes that the baseline land use forecast is "correct." This baseline forecast, however, is not based on the accessibility measures that are used to forecast incremental changes in development.
• The amount of existing development to re-allocate was selected based on judgment. The actual "mobility" of current development may be affected by a variety of factors.
• As applied by CDTC, there is no iteration or "feedback" between the transportation and land-use models. Ideally, the revised land use forecast resulting from congested traffic would be fed back into the travel model, which would then produce revised travel times, which would then be fed back into the land use model to produce a revised land use pattern, and so on until an equilibrium is reached. A single iteration, however, may be sufficient to gauge impacts at a sketch-planning level. Multiple iterations of the model could be tested to the extent that resources are available.

A discussion of other strengths and limitations of various land use models can be found in NCHRP Report 423A. Despite these limitations and approximations, the CDTC model probably gives a reasonable "first-order" approximation of impacts. Furthermore, it appears that planners and policy-makers in the Albany region found the results both believable and useful in assessing the potential effects of "what-if" scenarios for the region.

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