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16 S is the distance between the edgeline and the obstacles on centerline marking for lighting both high-speed and low-speed the inside curve in meters, and exits. This method had not yet been adopted in the United N is the required minimum number of markers visible to the States because of concern over the possibility of mistaking a road user; a minimum N of 5 is recommended. low-speed exit for a high-speed exit. To address these concerns, the author recommended: (1) modifying the type L-829 signs Bringing all of these principles together, the Dutch provide located at exits from the runway to increase their conspicuous- a detailed example application for a horizontal curve section. ness, (2) improved shielding to taxiway edge lights, (3) use of The reader is referred to the original citation for details (Rec- asymmetric instead of symmetric lenses on straight stretches, ommendation . . . 2005). (4) dimming of taxiway edge lights to reduce the "sea-of-blue" effect, and (5) use of high-efficiency retroreflective paint to Finally, the Dutch developed a multistep decision tree to mark the turn-offs to the exit taxiway to improve nighttime support implementation decisions related to IPM systems. In guidance. A system of pulsating blue lights at the entrance to summary, this multistep process the exit taxiway throat also showed promise (Douglas 1978). Distinguishes between roads that are already lit and unlit, More than 15 years later, Katz and Paprocki (1994) devel- Investigates the potential to remove or omit conven- oped and tested the performance of a prototype enhanced vi- tional lighting if IPM systems are installed, sual taxiway identification system, consisting of a segment of Investigates the potential to reduce the energy consump- green lights imbedded within the conventional runway cen- tion and environmental interference if IPM systems are terline lighting system at the FAA William J. Hughes Tech- installed, nical Center. Results of the effort indicated that the system Determines the cost-effectiveness of the IPM system and may be expected to provide enhanced and effective identifi- determines acceptability, and cation of taxiway exit locations at minimal cost. Investigates the potential to "switch off" conventional lighting under favorable traffic and weather circum- At the same facility, Gallagher (2001) evaluated the use of stances (i.e., as a cost-effective alternative to IPM sys- LED light strips with a focus on pilot and lighting personnel tem installation). perceptions. The LED light strips augmented painted surface markings, which were still deemed necessary for daytime and inclement weather conditions. Gallagher found that all HISTORICAL APPLICATIONS but one participant rated the LED light strips as a valuable augmentation to the painted surface markings. As mentioned previously, IPM systems were first used to provide path guidance for airport runways and taxiways and Most recently, Patterson (2004) reported specific opera- later emerged as an enhanced warning tool for pedestrian tional problems attributable the use of runway guard lights crosswalks. Although the technology characteristics and sub- installed at hold lines at the Chicago O'Hare International sequent costs of IPM systems have changed significantly Airport. In this application, a series of alternate flashing, yel- since these earlier applications, a review of experiences re- low, unidirectional, in-pavement lighting fixtures are equally lated to IPM system installation and maintenance may provide spaced along a runway holding position. These markings are valuable precautionary information for new system installa- intended to be visible only to aircraft approaching the hold tions. In addition, a review of observed IPM system effective- position from the taxiway. In some instances, however, pilots ness may support decision making for related applications. have reported that the lights are visible from the opposite side For instance, it might be inferred that intersection stop bars of the fixtures (i.e., to aircraft exiting the runway) resulting in equipped with IPM systems may experience similar benefits false guidance information to the pilots. No information was related to reduced vehicle approach speeds and increased ve- provided regarding how this issue was resolved. hicle compliance as those observed for pedestrian crosswalks. These limited reported experiences suggest important find- ings related to directional illumination, luminous intensity, Airport Runways and Taxiways and supplemental use of surface markings. Installation, main- Published information related to the use of IPM systems on tenance, and cost information was not uncovered (except for airport runways and taxiways focused predominantly on the the earliest types of IPM systems). The transferability of this evolution of technology from tungsten bulbs, which were latter information is likely more limited given differences in expensive to install and maintain, to LED light sources, which vehicles and vehicle operating characteristics between road- were found to be less expensive to install and operate. A few way and airport environments. studies were uncovered that focused on the performance of IPM systems in airport applications. Pedestrian Crosswalks As early as 1978, Douglas investigated the use of green Commonly referred to as "flashing crosswalks," IPM sys- lights installed in the runway surface on the extended taxiway tems for pedestrian crosswalk applications include the basic

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17 reportedly experienced frequent water condensation and bro- ken filaments. Applying more generally to all IPM system marker types, recessed markers require frequent cleaning to eliminate dirt and debris from the lens surface. In-pavement markers that protrude above the ground have experienced damage by street cleaners and snowplows (in at least one application, the damage did not prevent the light from re- maining operational) (Malek 2001). Manufacturers moved to aluminum or stainless steel housing materials to address this issue. Activities such as street repair or resurfacing require the IPM system to be removed and reinstalled or lost. Challenges related to system settlement have also been reported; over time and under traffic load, the markers are pressed further into the pavement, eventually damaging the power supply conduit and causing system failure (Ericksen FIGURE 4 "Smart crosswalk system" (Courtesy: LightGuard 2007). City officials in Santa Monica suggest that the use of Systems, Inc.). portland concrete cement instead of asphalt concrete pave- ment would address this challenge (Ericksen 2007). components of: (1) IPMs, (2) an AC or solar power source, and (3) a manual push-button or passive activation system City officials in Santa Monica, California, also report (see Figure 4). Over time, and with expanded IPM system significant delays in receiving system parts (e.g., replace- implementation, various installation, operation, and mainte- ment lights) when system failures do occur. These reported nance challenges have been identified. delays differ between independent suppliers. One supplier at- tributed delays to the discontinuance of the product, whereas As noted previously, some passive activation systems, par- another attributed delays to pending product improvements ticularly those relying on microwave detectors, have experi- that would result in a brighter, and more robust, marker and enced higher rates of false positives and misses. A passive a backlog of replacement orders resulting from a minor engi- activation bollard system in San Jose, California, malfunc- neering design change (Ericksen 2007). tioned as a result of vandalism (Malek 2001). Of more con- cern, citizens in Santa Monica, California, are reporting a For pedestrian crosswalk applications, IPM system costs false sense of pedestrian security, which, when combined have ranged from $5,000 to $100,000 per application. Factors with a high rate of system malfunction, has purportedly led to affecting cost include the length and layout of the application multiple pedestrianvehicles crashes and one resulting death and the subsequent number of markers required; specific fea- (Ericksen 2007). tures of the IPM system (e.g., unidirectional or bidirectional dis- plays and operational modes); the availability and nature (e.g., A greater variety of maintenance challenges have been solar) of power at the site; the condition of the pavement and identified. Specific to halogen light sources, halogen lamps any remedial actions required before IPM system installation; TABLE 2 BENEFITS OF IN-PAVEMENT WARNING LIGHTS AT CROSSWALKS Measure Author Comments Enhanced Driver Huang et al. (1999) Actuated flashing provides an advantage over Awareness continuous flashing Malek (2001) More effective than overhead beacon, especially at night Boyce and Van Derlofske (2002) Whitlock and Weinburger (1998) Particularly beneficial during adverse weather Increased Vehicle Huang et al. (1999) Yielding Reduced Vehicle Huang et al. (1999) Speeds Boyce and Van Derlofske (2002) Prevedouros (2000) 17.8% to 16.2% reduction in maximum speed 27.2% to 25.2% reduction in average speed 16.3% to 14.0% reduction in 85th percentile speed Reduced Vehicle/ Huang et al. (1999) Pedestrian Conflicts Boyce and Van Derlofske (2002) Reduced number of vehicles entering crosswalk while pedestrian waiting Lower Pedestrian Prevedouros (2000) 50.5% reduction (26.7 s to 13.2 s) Wait Times

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18 and traffic control requirements. A broader range of costs is vehicle and a pedestrian in a crosswalk at the same time). anticipated for the emerging IPM system applications intended A single study was identified that considered the effect of IPM to enhance guidance, regulation, and illumination. systems on pedestrian crosswalk operation (i.e., in reducing pedestrian wait times). Table 2 summarizes previous studies Regarding the effectiveness of IPM systems in pedestrian investigating the effectiveness of IPM systems at pedestrian crosswalk applications, the results are generally favorable al- crosswalks. though the quality of prior study designs has been criticized. Few studies have directly measured the effect of IPM systems Despite individual study limitations, a positive trend in on pedestrian crosswalk safety; the infrequency of crashes IPM system effectiveness in enhancing pedestrian crosswalk and the time duration required to achieve an adequate sample safety and operation can be observed. In-pavement marker preclude direct measurement. Instead, prior studies have con- systems have generally been shown to increase vehicle driver sidered various surrogate safety measures including enhanced awareness, increase vehicle yielding, reduce vehicle approach driver awareness, increased vehicle yielding, reduced vehicle speeds, reduce vehiclepedestrian conflicts, and reduce pedes- speeds, or reduced vehiclepedestrian conflicts (defined as a trian wait times in this type of application.