Parametric functions for conceptual and feasibility estimating in public highway project portfolios
Owners face challenges in setting priorities between potential projects to maintain, rehabilitate, and improve their infrastructure. The estimated cost of each potential project is a factor that owners use in setting priorities between projects and in developing their long-term maintenance and construction project portfolio. Owners face a dilemma: considerable effort is needed to develop accurate estimates of the cost of each project, but this effort will be wasted if the particular project is not selected for the long-term plan. They therefore need estimating methods that will enable them to develop reasonably accurate early stage cost estimates without an excessive amount of effort. These early stage estimates are “conceptual cost estimates” and “feasibility cost estimates.”
This research examines the tools that are available to owners for performing early stage cost estimates for infrastructure projects. It then compares alternative parametric functions that could be used for that purpose, using data from public agencies in California. These functions are the linear parametric, common exponential parametric, and modified Cobb-Douglas exponential parametric models.
This research tests the models on 1 common type of project, pedestrian access facility projects on highways. In the United States (US) these projects result, directly and indirectly, from the Americans with Disabilities Act (ADA) that Congress passed in 1990. On highways, they produce three types of improvement: 1. wheelchair ramps at street corners to allow people in wheelchairs to cross streets at designated pedestrian crossings, 2. wheelchair-accessible sidewalks, and 3. audible signals at signalized intersections to inform visually impaired people when a pedestrian signal is in their favor.
The author developed a data set of 39 pedestrian access facility projects on state highways in California, used multiple regression analysis to find 4 best-fit versions of each of the 3 functions (i.e., 12 alternatives in all), and evaluated them using the Choosing By Advantages (CBA) method.
The author then benchmarks the preferred state highway cost estimating model identified in the CBA against 10 city-street pedestrian access facility projects that had been completed by 4 cities in the San Francisco Bay Area. He finds a significant difference between the state highway project cost data and the city street project cost data, and further rationalizes that these differences have their roots in both the contracting methods used by the agencies and the fact that Caltrans prepares detailed designs while cities do only minimal design. The data suggests that there is an opportunity to increase output and lower the costs of pedestrian access projects (and perhaps other types of highway projects as well) by decreasing the Caltrans design effort and transferring more of the design effort and consequent risk to contractors. This could be tested through experimentation on selected pedestrian access facility installations.
This dissertation contributes to knowledge by providing a review of the place of conceptual and feasibility estimating both with respect to the overall project timeline and with respect to the methods used. It provides specific examples of the use of the various classes of estimates in the development of highway projects, and it provides a synthesis of the research on conceptual and feasibility estimating methods, most notably of parametric estimating. It then provides specific examples of parametric estimates on pedestrian access projects on California State Highways and in San Francisco Bay Area cities. Finally, it unveils the successful use by Bay Area cities of a minimal amount of design when developing design-bid-build contracts for pedestrian access facilities.
The dissertation aims to provide an approach that could be used both for project-by-project conceptual estimating prior to the start of work on highway projects and for evaluating the overall credibility of the estimates on large portfolios of highway projects.