Deploy a variable speed limit system only after the software systems required to support it are mature and reliable.
Experience from iFlorida Model Deployment
- Identify statutory and regulatory speed limit requirements before considering the use of variable speed limits. Statutory restrictions limit the applicability of VSL on I-4. The minimum speed limit of 40 mph, combined with the normal speed limits of 50 and 55 mph on I-4 in Orlando, meant that speed limits could be varied only over a small range. The requirement for an engineering and traffic investigation before speed limits could be changed was also problematic. FDOT determined that it could perform this type of investigation once to identify the types of traffic conditions that would warrant lower speed limits, and RTMC staff could then change speed limits after verifying that the specified conditions for lowering speed limits had been met.
- Develop the concept of operations for VSL before designing the VSL system and validate it against historical data. Because of the long lead time for deploying the iFlorida field equipment, FDOT contracted to deploy the VSL signs before completing the VSL concept of operations, specifying that the signs be deployed in the area where congestion most often occurred. The VSL concept of operations emphasized the benefits of lowering speed limits upstream from a congested area. A comparison of the VSL sign locations to historical patterns of recurring congestion indicated that the signs were to be located to cover the area upstream of the points where eastbound congestion typically began, though not the area upstream of the full extent of the congestion at its peak. The VSL signs appeared to be west of the area congestion typically occurred in the westbound direction. FDOT noted that the agency had discussed each of these issues prior to selecting the site for deploying the VSL signs; however, because these issues were not addressed in the concept of operations document, the basis for the VSL design was not documented.
- Ensure that the algorithms for recommending speed limit changes can detect and correct for low vehicle speed observations that are not related to congestion. A review of historical speed data identified a number of cases where low speed measurements did not appear to be related to congested conditions. In some cases, this appeared to be caused by a faulty detector (e.g., the detector consistently reported low speeds). In others, a single low value would be embedded in a series of otherwise normal values. In either case, the algorithms for recommending lower speed limits should include methods to detect and disregard low vehicle speed measurements that are not related to congestion. (In the case of iFlorida, the problems with the CRS made this very difficult or impossible to do.) Problematic detectors might also be taken offline until repaired, in which case the algorithms must be able to adapt to the fact that detectors may be taken offline. The robustness of those algorithms could be verified by applying them to historical data.
- Require operator approval of all speed limit changes. FDOT's design of the VSL system called for an automated system to monitor traffic and weather conditions and recommend lower speed limits when conditions appeared to warrant them. RTMC operators would be required to investigate traffic conditions and approve the recommendation only when warranted by current traffic conditions. Since any algorithm for recommending speed limits is likely to fail on occasion, this approach will prevent those failures from resulting in changing speed limits at inappropriate times.
- Deploy a VSL system only after the systems required to support it are mature and reliable. Because of its participation in the Model Deployment and the required milestones, FDOT had to deploy infrastructure and develop the operating system concurrently. This resulted in the deployment of VSL signs before the CRS software for supporting those signs had proven reliable. These signs remained set at the fixed speed limits when the initial version of the CRS software did not operate reliably and 2 years of additional work by the CRS contractor did not remedy the problems. Thus, FDOT had to bear the cost of deploying VSL signs without obtaining the benefit of using variable speed limits.
Author: Robert Haas (SAC); Mark Carter (SAIC); Eric Perry (SAIC); Jeff Trombly (SAIC); Elisabeth Bedsole (SAIC): Rich Margiotta (Cambridge Systematics)
Published By: United States Department of Transportation Federal Highway Administration 1200 New Jersey Avenue, SE Washington, DC 20590
Source Date: 01/30/2009
EDL Number: 14480URL: http://ntl.bts.gov/lib/31000/31000/31051/14480.htm
Average User Rating
VSL, managed lanes