When nighttime lane closures were needed for I-35 that would likely create a traffic queue, TxDOT combines portable work zone ITS concept with portable rumble strips into an end-of-queue (EOQ) warning system.
Texas, United States
Safety Effects of Portable End-of-Queue Warning System Deployments at Texas Work Zones
Summary Information
An ongoing widening effort on Interstate 35 (I-35) through central Texas prompted TTI to assist TXDOT with implementing an end-of-queue (EOQ) warning system to help mitigate upstream end-of-queue crashes at work zone lane closures. The system consists of a highly-portable work zone ITS concept based on easily-deployable radar speed sensors (to minimize calibration requirements) that are linked to one or more portable changeable message signs (PCMS). Additionally, TxDOT opted to combine the portable work zone ITS concept with portable rumble strips whose tactile and auditory stimuli were estimated to be beneficial in gaining the attention of distracted drivers approaching a work zone lane closure.
Safety Analysis Methodology
The analysis used the "with-EOQ" and "without-EOQ" lane closure nights as "treatment" and "control" group datasets, and compared crashes occurring during both sets of nights to what would have been expected on those nights if the corridor had not been under construction. To accomplish this, the corridor was divided into a set of discrete homogenous segments and Empirical Bayes analysis methodologies were applied for years 2003-2009, using the Enhanced Interchange Safety Analysis Tool (ISATe). The end result of that analysis was an expected number of crashes/year and crashes/night in each segment. The average proportion of crashes occurring during the same nighttime hours that lane closures were allowed during construction was then calculated from the calibration data, and applied to each of the segments to obtain the expected nighttime crashes/day. Next, the expected crashes per night (ECN) for each lane closure where queues were expected to have occurred (with and without EOQ) was determined. The lane closure section analyzed included a queue section that extended from five miles upstream of the beginning of the lane closure and the length of the lane closure. The discrete homogenous (ISATe) segments corresponding to that lane closure and queue section was determined, and the expected number of crashes from the ISATe analysis corresponding to that lane closure section determined.
Analysis Findings
- The EOQ warning system was estimated to have reduced crashes 44 percent from what they would have otherwise been if the system had not been used.
- The crashes that did occur were less severe, most likely because fewer of them were of the high-speed rear-end collision variety.
- Using traditional societal crash cost values updated to 2014 dollars, the use of the EOQ warning system at nighttime lane closures reduced crash costs by $1.36 million over the one year analysis period. This equates to $6,313 in crash cost savings per night of deployment.
- Compared to the approximate costs of procurement and deployment of these systems, a break-even point is achieved after 95 to 190 nights of use.
