Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
8Measuring Sight Distance and Other Critical Information The purpose of this chapter is to provide users with instructions on collecting field data for the available ISD as well as other relevant data needed to use the guidance. The focus of this guidance is on existing intersections; however, the same field-collected values described here could be determined as part of the design process for a proposed intersection. There are two different scenarios in which the guidance could be used at the design stage. In the first, changes are proposed to the design of an existing intersection or the surrounding environment. This may include removing an obstruction that has been identified either by the public as a hazard or by a high crash rate. The second is when consider- ing alternatives for a new or reconstructed alignment or intersection. There are three primary areas of focus in this chapter: ISD, traffic volumes, and intersection characteristics. Each subsection includes detailed instructions on the best practices for data collection, measure- ment methods, and other information needed for determining a CMF. The information presented in this chapter was developed based on the results of an extensive evaluation and data collection effort aimed at identifying a method to cost-effectively yet consistently collect ISD, volume, and intersection characteristics in the field. Additional detail is published in NCHRP Web-Only Document 228. C h a p t e r 2 Practitioner Tips: Recommended Equipment ⢠Laser rangefinder with 4x magnification scope mounted 42 in. above the ground using a monopod or similar. This is the sighting height. ⢠Metal target that is 14 in. by 14 in., such as a slow-moving-vehicle sign or sign similar in size and color, mounted on a post or stick so that the target can be positioned 42 in. above the ground. This is the target height. ⢠A measuring wheel. ⢠Safety vests for all members of the field collection team. ⢠Cell phones, two-way radios, or similar devices for the field collection team members. ⢠Any necessary traffic-control measures (such as portable survey crew signs or flaggers), depending on the characteristics of the intersection and the operating procedures of the collecting agency. Tool Box: Standard ISD The CMFs/CMFunctions in this guidance are based on the ISD measured at a point on the minor road that is set 14.5 ft back from the edge of the closest through- vehicle travel lane on the major road, as described in the Green Book (AASHTO 2011).
Measuring Sight Distance and Other Critical Information 9 Data Collection Approach Two staff members are required to collect ISD in the field using the approach described here. One person is called the targeter, and the other is the sighter. The two-person team assembles the required equipment before heading out to the field. Data collection starts at the intersection by the sighter measuring a point on the minor-road approach that is 14.5 ft back from the edge of the closest vehicle travel lane of the major road and 4 ft to the right of the centerline of the minor-road approach. The Green Book refers to this as the decision point, as previously defined (AASHTO 2011). This point is illustrated in Figure 2 and represents the location from which drivers on the minor approach, stop, and view traffic to select an appropriate gap and enter the intersection. The sighter is positioned at the DP with the laser range finder placed 42 in. high using a mono- pod or similar device. The targeter walks away from the intersection on the shoulder of the major road in the direction that is being measured carrying the metal target and the measuring wheel. The targeter stops periodically to communicate with the sighter over the radio about the sight distance (e.g., âcan you still see me?â). The targeter stops when the point is reached where the full metal target (positioned in the center of the nearest major-road through lane) just begins to disappear from the view of the sighting person while looking through the rangefinderâs scope. This location becomes the critical point (CP) for that direction of view. If the major road between the CP and DP is characterized as a tangent (i.e., it lacks horizontal curvature), then the rangefinder is used to measure the distance between the two. This is the ISD. If the major road includes one or more horizontal curves between the CP and DP, then the targeter uses the measuring wheel to measure the ISD by walking along the major road from the CP to the DP. If the target is still in view beyond a quarter of a mile (1,320 ft) from the DP, the ISD is con- sidered more than sufficient. This establishes a practical limit for field collection. The field collection process is summarized in Figure 3. All field personnel should exercise caution when collecting this information. If traffic volumes or other conditions render it unsafe to position the equipment at the intended locations of the CP or DP, then the data collectors can: ⢠Reposition themselves as close to the intended positions as possible, ⢠Employ traffic control measures for a short period of time while the ISD is measured, or ⢠Use a three-person field team with one person working as a spotter for traffic. MAJOR ROAD 14.5' 4.0' M IN O R RO A D Figure 2. Decision point.
10 Guidance for evaluating the Safety Impacts of Intersection Sight Distance Traffic Volumes The safety effect of ISD is influenced by two-way AADT and the speed limit on the major street. Generally, the number of crashes increases as the major-road and minor-road volumes increase. Safety performance is expected to be more sensitive to ISD when the major-road AADT and speed limit are higher. To use the CMF graphics in Chapter 3 of this guidance, major-road traffic volumes and speed limits are needed. AADT is needed for the major road; however, a detailed turning movement count is not needed. There are two overarching strategies for gathering traffic volumes: use of existing data sources and data collection. Traffic volumes can also be estimated for design applications or future analysis. Existing Data Sources Practitioners are encouraged to access local or state databases before exploring options for field data collection. Many states, metropolitan planning organizations, rural planning orga- nizations, and regional transportation planning organizations, as well as some local agencies, maintain detailed GIS maps of traffic volumes expressed as AADT or, potentially, vehicle miles traveled. These may be available in online databases, maps, or other documentation. Collecting Traffic Volume If no existing source has volume data available, a short-term traffic count can be conducted to collect volume on one or both roads. The ITE Manual of Transportation Engineering Studies (Schroeder et al. 2010) provides a detailed description of several methods to collect traffic vol- umes, including information on how to convert a short-term count to AADT. Crash History For the intersection of interest, 3 to 5 years (5 years preferred) of crash data are necessary for safety analysis. Each multiple-vehicle crash occurring during the identified years should be reviewed to determine: ⢠Initial approaches for vehicles involved in the crash, ⢠Travel directions for vehicles, and ⢠Maximum severity of injuries to drivers or occupants involved in the crash. DP Figure 3. Data collection approach for measuring intersection sight distance.
Measuring Sight Distance and Other Critical Information 11 The reviewer should also identify target crashes using these criteria: ⢠Whether the location of the crash has a milepost within 250 ft of the intersection, and ⢠Whether the crash involved a vehicle on the major road and a vehicle on the minor road. ISD is considered and analyzed by approach direction (or directional analysis unit), and therefore target crashes should be identified by the directional analysis unit that is being con- sidered. Figure 4 provides an example three-leg intersection. If a vehicle on the minor-road approach collides with the vehicle approaching from the left, then the crash is associated with the left-directional analysis unit. For four-leg intersections, there are a total of four analysis units considered. Each analysis unit is associated with an ISD value for crash analysis. It is impor- tant to locate the crashes properly to determine the impact on crash frequency of changing the specific ISD. Crash severity is used to further identify injury crashes (K, A, B, and C on the KABCO scale) as a subset of target crashes. All crash types should be summarized by associated minor-road approach and major-road approach. The summary may consist of the sum of crash counts over a given period or may consist of crash frequency (crashes per year). Crash diagrams are useful tools for consistently characterizing and visually representing crashes. Due to the complexity of analysis, crash diagrams focused on target crashes will help the practitioner identify the appropriate approach direction for each crash, especially for four-leg intersections. Since target crashes are relatively rare events at stop-controlled intersections, each crash can be represented individually, with the severity symbolized for each. Target crashes can Practitioner Tips: Other Considerations There are many factors that affect safety at an intersection. While in the field measuring ISD, the data collection team may also want to make other observations, including use of photographs and an intersection sketch for further discussion in the office. Relevant observations include: ⢠Posted speed limits; ⢠Approach speeds (e.g., 85th percentile speeds); ⢠Intersection configuration, including number of lanes and the presence of turn lanes; ⢠Observations of vehicle type (e.g., presence of slow-moving farm vehicles or many trucks); ⢠Intersection skew; ⢠Observations of driver behavior that may affect safety (e.g., rolling stops); ⢠The ability to judge the speed of approaching vehicles (i.e., are there sufficient reference points so that drivers at the DP can gauge the speed and position of approaching vehicles); ⢠Photographs of obstructions to sight distance such as fences, trees, signs, and buildings; ⢠Traffic control device visibility and conspicuity; and ⢠Evidence of near miss or unreported crashes (e.g., skid marks or the presence of debris.)
12 Guidance for evaluating the Safety Impacts of Intersection Sight Distance be placed on the schematic by approximate location, grouped by accident type, or grouped by intersection sight distance association. Crash diagrams may also be used to help verify whether crashes were target crashes. Figure 5 is an example crash diagram for a three-leg intersection that graphically depicts all crashes in a given time frame. In this example, there are two potential ISD triangles to consider, left- and right-looking from the northbound minor road. It is important to carefully review Figure 4. Three-leg intersection for minor-road approach. Figure 5. Example crash diagram.
Measuring Sight Distance and Other Critical Information 13 all crashes to determine which are target crashes associated with ISD and which may be single- vehicle crashes or do not involve a vehicle from the minor road. There are four target crashes in this example associated with the left sight triangle, as represented with two arrows pointing at each other. Additionally, the crash reports can further identify whether crashes are due to sight distance or gap-acceptance issues, or whether the driver failed to obey the traffic control device. Review- ing the vehicle speeds at impact can help to identify intersection sight distance issues. However, if the intersection is being considered for changes, all target crashes or target fatal and injury crashes should be included in the CMF-based analysis, regardless of ISD relation.
