Newsletter - Summer 2019
The National Environmental Policy Act (NEPA) process is an approach to balanced transportation decision-making that requires the examination and avoidance of potential impacts to the social and natural environment. This process allows the best overall alternative to be chosen based upon a balanced consideration of the need for safe and efficient transportation; of the social, economic, and environmental impacts.
During the Alternative Development Process a range of alternatives will be considered and evaluated, including the no-build option, mass transit, high-occupancy vehicle lanes, and ridesharing. The alternatives analysis will explain how alternatives were evaluated and why each alternative was eliminated from consideration. The range of alternatives and results of the analysis will be presented for your review and comment in the Fall of 2019.
Exploring What’s Underneath through a Geotechnical Investigation
The project team has been hard at work completing the geotechnical investigation, both on land and under water, for the US 278 Corridor Improvements project. A geotechnical investigation includes the exploration of the surface and underground conditions of a site and is generally completed at the beginning of a project. This investigation provides a better understanding of the existing physical conditions of the project site and may inform the team of any potential challenges or limitations to consider when designing a roadway or bridge.
For this project, a diving service helped locate and protect underwater utilities in both Skull and Mackay Creeks, while the geotechnical team collected traditional soil samples. Currently, laboratory testing of samples collected, including those gathered from both land and underwater locations, is underway. Additionally, the site specific soil p-y method, a test to analyze the ability of local soils to resist pressure and provide support for project features, such as roads and bridges, is in progress.
This data will be used to develop seismic models that will provide valuable information about the existing bridges as well as the needed information for the design of the new Mackay Creek bridge. Lastly, this same geotechnical data will be used to calculate potential settlement and overall seismic stability of the roadway approaches to ensure the improvements can withstand specific seismic thresholds.
Improving Traffic Along US 278 through Growth Modeling
Traffic grows in relationship to several factors such as: population growth, changes in employment and economic development opportunities, tourism activity, and the movement of goods in to, out of, and through a region. All these factors play a role in estimating future traffic. Understanding how traffic volumes have changed over the past five years is beneficial to understanding the current environment; however, historical growth trends do not tell the full story of future traffic conditions.
This project has a horizon year of 2040, and the project team used the Lowcountry Area Transportation Study (LATS) Regional Travel Demand Model to help estimate what future traffic would look like along US 278 and the surrounding roadways. The regional model, which includes data for Beaufort, Colleton, Jasper and Hampton Counties, is used to forecast future traffic because it includes information such as estimated population and employment growth and the influence of tourist activity on traffic and travel patterns on a regional basis.
To account for the consistently high summer tourist volumes, while balancing the off season, the project team determined that the “30th highest day” volumes would be used for the analysis of the intersections. For example, if you ranked all the traffic volumes (number of cars on the road) for a year from highest to lowest, the number of vehicles estimated on the 30th from the top would be used to assess the transportation needs. This value represents more cars on the road than under “average daily” conditions. Additionally, it accounts for the seasonal tourists without over-designing the roadway for the busiest day of the year, which could result in constructing too many lanes and using more public infrastructure dollars than appropriate for the community’s needs. This method is consistent with traditional traffic engineering practices and accounts for the fluctuation in traffic conditions throughout the year in a community that experiences heavy tourist traffic.
Through the ongoing traffic studies, the project team is making determinations about how many lanes are needed through the corridor, as well as the traffic operations improvements that may support the safe and efficient movement of cars. In other words, the traffic engineering team is working to reach a balance between widening roads and managing traffic through improved traffic signals, access management, and signage.
Where We Have Been
In February, the project team attended the Gullah Celebration Market to provide information about the project and learn more about the Hilton Head Island and Bluffton communities. In March, we held a meeting with area stakeholders to understand the unique concerns of the local residents and businesses and to share insight into the project development process. We are using the information heard at these events to put together the Community Impact Assessment to understand the impacts the project may have on the community and to inform the roadway improvements design process.