Ongoing Research

HSIS project staff conduct an ongoing program of research. Research topics are chosen by the FHWA Office of Safety Research and Development and FHWA Office of Safety as critical to their missions. An updated list of current projects is being compiled and will be available soon.

Planned HSIS Safety Evaluations

Converting Traffic Signals from Incandescent to LED Bulbs

Across the nation, many agencies are beginning to convert traffic signals to LED bulbs. Much of this is driven by a 2006 directive from the Department of Energy that stipulated that traffic signal manufacturers must meet ENERGY STAR power requirements, which effectively requires the use of LEDs. NCHRP web document 146 indicates that LED signals may have the ability to increase intersection safety because their low power requirement allows for the use of a battery backup to keep the signal operational during power outages (Bullough et al., 2009). Although LEDs are mainly installed in order to reduce energy consumption, not much has been done to investigate the safety effects of this conversion.

One of the only safety-focused studies on LED effects was published in a recent ITE Journal article. Eustace et al. conducted an exploratory study in Middleton, Ohio to determine the safety effects of LED conversion (Eustace et al., 2010). The team identified only 10 intersections that met the study criteria — 8 of which were conversions and 2 of which were reference sites. A before/after EB analysis was conducted and the researchers concluded that crashes increased by 71% after LED installation. The authors acknowledge that the low number of treatment sites and small reference group result in less than conclusive findings.

The objective of this study is to determine if the conversion of traffic signals from incandescent to LED bulbs had an effect on crashes, and, if so, to quantify that effect in terms of a crash modification factor. The analysis will use data from Charlotte — our HSIS city. In 2008 Charlotte DOT contracted a firm to change over the majority of the city's signalized intersections from incandescent to LED bulbs. Contractors were instructed to change out all bulbs at an intersection, even if LEDs were already present. Roughly 90% of signalized intersections were converted during the life of the contract — 2008 to 2009.

Safety Effects of Intersection Angle

The preferred design for intersections includes adjacent legs that intersect at 90 degrees. However, there are occasions where physical constraints result in intersection angles less than 90 degrees, resulting in skewed intersections. The smallest acute angle between the intersecting legs is the intersection angle. Skewed intersections create safety problems for both motorists and non-motorists. It is more difficult for drivers to detect and judge the speed of approaching vehicles on conflicting paths. Skewed intersections also have a larger footprint, which makes the conflict zone larger for all modes and increases the walking distance for pedestrians on specific legs. Finally, the intersecting legs with acute angles between them can adversely affect the turning radius, making it more difficult to accommodate large trucks.

The AASHTO Policy on the Geometric Design of Streets and Highways — the Green Book (AASHTO, 2004) recommends that intersection legs meet at 90 degrees wherever practical and never less than 60 degrees. The ITE Traffic Engineering Handbook (ITE, 2009) provides similar guidance. However, prior ITE guidance documents recommended a minimum intersection angle of 75 degrees (ITE, 1999; ITE, 1984). The FHWA Highway Design Handbook for Older Drivers and Pedestrians also recommends right angle intersections where right-of-way is not constrained and minimum intersection angles of 75 degrees where right-of-way is restricted (Staplin et al., 2001).

The objective of this study is to derive quantitative relationships between intersection angle and safety. The relationships will be used to:

• determine if there is a critical minimum angle at which safety is substantially diminished (75 degrees, 60 degrees, or other),
• determine appropriate crash modification factors for reducing or eliminating the skew angle of an intersection, and
• assess the need for revision of current geometric design policies and practices.

Data from MN will be used in that analysis. The HSIS data for MN includes an intersection file with some information on intersecting angle (yes or no). These data will be supplemented with data from digital images to acquire the magnitude of the intersecting angles. The scope of the effort will be limited to 3-leg and 4-leg stop-controlled intersections in both rural and urban areas.