دانلود رایگان مقاله حذف تنگنای اتوبان و اثرات بعدی در حجم راه آهن و آزادراه

ABSTRACT

The removal of a freeway bottleneck in California has allowed researchers to investigate short-term induced travel and a potential mode switch from rail transit. This particular bottleneck, a double lane drop from reduced tunnel capacity, is a unique case as alternate auto routes are quite undesirable; the only other option is to consider nearby rail transit. Freeway volumes and rail ridership were examined before and after the removal of the bottleneck to estimate the extent of induced travel. Freeway volumes during both commute periods increased 10–13%, faster than other nearby locations, and rail transit ridership between stations on either side of the pre-existing bottleneck showed modest declines despite system wide increases. Differences of means testing confirmed that many of these changes were statistically significant. Examining the magnitude of induced travel is relevant when making policy decisions for removing mature bottlenecks that involve the use of public finances.

4.1. Policy implications and conclusions

The addition of the fourth bore of the Caldecott Tunnel has presented itself as an excellent example of the effects of bottleneck removal in regards to potential induced travel. In past research, the emphasis on induced travel has typically been on new construction and freeway expansion, sometimes in areas of rapid growth. This case study however has looked at bottleneck removal on a mature freeway that had been largely unchanged for nearly 40 years in an area that has not had significant population growth since the 1980’s. Nevertheless, just one year after the new tunnel was finished there were already obvious signs of short term induced travel; in the westbound direction nearly 2300 extra vehicles in a typical afternoon, or about 375 vehicles per hour. The first policy implication is quite clear; there will need to be an expectation of new demand from endogenous sources even in a mature setting. Today, when most agencies seek to remove existing bottlenecks, the presumption is that in a mature situation the existing traffic pattern will not significantly change and induced travel will be minimal. This case study has proven otherwise that people will still change in the short-term even if the status quo has been in place for decades. As such, for other types of bottleneck removal there could be other regional changes, possibly on local streets, that are not accounted for. All across the United States and beyond, there are plans to remove longstanding freeway bottlenecks to improve travel time and reduce congestion. In the San Francisco Bay Area, Caltrans is currently removing another famous lane drop freeway bottleneck in Marin County on US 101 known as the Novato Narrows project. Proposed construction will widen a short four lane section to meet the six lane capacity of the freeway on either side. While the removal of the narrow section on US 101 will surely reduce congestion in the short term, as it did in this case study on SR 24, there will likely be unanticipated volume increases. Furthermore, the county is also currently constructing a regional rail transit system, and ridership projections may be generously high if congestion is removed on the adjacent freeway at the same time. Similarly, in Los Angeles Caltrans is working on a 1.3 billion dollar project to widen I-5 from downtown to the Orange County line, lessening the effect of a 50 year old lane drop when entering Los Angeles County in the northbound direction (Molina, 2013). As evidenced by the Caldecott Tunnel project, not only will this bottleneck removal result in increased volumes on I-5, it could also create a mode switch from nearby Metrolink regional rail and Amtrak California, negating the environmental gains from reduced automobile congestion. Aside from the appearance of induced travel from the removal established bottlenecks, this effect might also be seen when lane closures are removed after a significant amount of time. Even if work zone congestion causes drivers to seek alternate routes, it’s highly possible that these drivers will return even if the timeline is many years.