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FHWA Home / Safety / Local and Rural Road / Intersection Safety: A Manual for Local Rural Road Owners

Intersection Safety: A Manual for Local Rural Road Owners

Table of Contents

3. Safety Analysis

Conducting safety analyses will assist the practitioner in identifying intersections with safety issues and selecting countermeasures to improve them. The types of analysis can be qualitative or quantitative. This section outlines steps to identifying intersections with safety issues and making data-supported decisions as to the type, deployment levels, and locations of countermeasures. These steps build on the previous discussion of overall safety implementation approaches and sources of information for identifying safety problems. Additional information on analysis procedures and data can be reviewed in "Road Safety Information Analysis: A Manual for Local Rural Road Owners."

3.1. Crash Frequency

Crash frequency represents the number of crashes that have occurred at a particular intersection over a period of time. It can be determined from the State or local crash database (or law enforcement crash reports).

This allows the practitioner to:

Once this information is collected and displayed, the local practitioner can compare intersections using cluster analysis to determine crash experience by frequency levels.

3.2. Crash Rate

Crash frequency alone is often inadequate when comparing multiple intersections or prioritizing locations for improvement. Crash rates can be an effective tool to measure the relative safety at a particular intersection. The ratio of crash frequency (crashes per year) to vehicle exposure (number of vehicles entering the intersection) results in a crash rate. Crash rate analysis can be a useful tool to determine how a specific intersection compares to the average intersection on the roadway network.

For example, it is possible that two intersections in a jurisdiction (Intersection A and Intersection B) each have a similar number of crashes. However, Intersection A may have many more vehicles entering the intersection on a typical day than Intersection B, as shown in Table 2. In order to effectively compare the safety of the two locations, the practitioner must factor in the level of exposure to crashes for each intersection. Exposure data here is represented by the number of vehicles entering the intersection. Population and number of licensed drivers within a jurisdiction are other types of exposure data that can be used depending on the circumstances and availability.

Crash rate is often used to prioritize locations for safety improvements when working with limited budgets to achieve the greatest safety benefits with available resources.

Crash rates can be calculated using the following widely accepted equation. This equation can be used for any crash type or severity. The intersection crash rate based on vehicles entering the intersection is calculated as:

Equation. Crash rate for the intersection (crashes per million entering vehicles) equals 1 million times the total number of intersection-related crashes in the study period divided by the product of 365 times the number of years of data times the traffic volume entering the intersection daily.

Where:

R = Crash rate for the intersection expressed as crashes per million entering vehicles (MEV)

C = Total number of intersection-related crashes in the study period

N = Number of years of data

V = Traffic volumes entering the intersection daily

This equation relies on traffic volume information. Actual and estimated traffic volumes are often compiled and kept by State highway agencies, local governments and property developers.

In the following example shown in Table 2, two intersections have approximately the same number of crashes but different entering traffic volumes. By factoring in traffic volume (exposure), the calculation indicates that Intersection B may be a more promising roadway for safety treatments due to its higher intersection crash rate (measured in number of crashes for every 1 million entering vehicles).

Table 2. Example of the Intersection Crash Rate Calculation
Location Intersection crashes (C) Entering Traffic Volume (V) Years of Data (N) Intersection Crash Rate (R)*
Intersection A 25 14,000 5 0.98
Intersection B 22 6,500 5 1.85
*Measured as the number of crashes per 1 million entering vehicles.


Intersection A

Equation. Intersection crash rate equals the result of 1 million times 25 diviced by the product of 365 times 5 times 14,000 equals 0.98 crashes per million entering vehicles.

Intersection B

Equation. Intersection crash rate equals the result of 1 million times 22 diviced by the product of 365 times 5 times 6,500 equals 1.85 crashes per million entering vehicles.

Action: Calculate the crash rates for intersections experiencing crashes in the jurisdiction, and then use that crash rate to prioritize locations for investigation and possible treatments.

Develop a database to record crash rate calculations over time for comparison with intersections that have potential safety issues in the future. This can provide practitioners with a jurisdiction-specific average intersection crash rate for varying situations.

3.3. Geometric Issues

The geometric design of intersections can create navigational problems for motorists, potentially contributing to crashes at these locations. Among geometric design elements, two specific issues can cause safety concerns: sight distance limitations and skewed geometry.

3.3.1. Sight Distance

Insufficient sight distance can be a contributing factor in intersection traffic crashes. Intersection sight distance is typically defined as the distance a motorist can see approaching vehicles before their line of sight is blocked by an obstruction near the intersection. The driver of a vehicle approaching or departing from a stopped position at an intersection should have an unobstructed view of the intersection, including any traffic control devices, and sufficient lengths along the intersecting roadway to permit the driver to anticipate and avoid potential collisions. Examples of obstructions include crops, hedges, trees, parked vehicles, utility poles, or buildings. In addition, the horizontal and vertical alignment of the roadway approaching the intersection can reduce the sight triangle of vehicles navigating the intersection.

It is important for approaching motorists on the major road to see side street vehicles approaching the Stop sign, and for minor road motorists to see approaching major road vehicles before entering the intersection. Poor sight distance can lead to rear-end crashes on the approaches and to angle crashes within the intersection because motorists may be unable to see and react to traffic control devices or approaching vehicles.

The area needed for provision of this unobstructed view is called the Clear Sight Triangle (see Figure 3).

Figure 3. Sight Distance Triangles for 4-Leg Stop-controlled Intersections9
Diagram of an intersection showing the intersection sight distance triangles for approaching left and right turning traffic on a 4-leg stop-controlled intersection. Sight triangles are 90 degree scalene triangles whose shortest side begins 15 feet from the edge of the nearest through lane.

The Intersection Sight Distance (ISD) is measured along the major road beginning at a point that coincides with the location of the minor road vehicle. Table 3 provides the recommended values for ISD, based on the following assumptions:

If conditions at the intersection being evaluated differ from these assumptions, an experienced traffic engineer or highway designer should be consulted to determine whether different ISD values should be used.

Table 3. Sight Distance at Intersections
Speed (mph) * Stopping Sight Distance (ft.) Design Intersection Sight Distance (ft.)
25 155 280
30 200 335
35 250 390
40 305 445
45 360 500
50 425 555
55 495 610
60 570 665
65 645 720
Source : A Policy on Geometric Design of Highway and Streets, 5th Edition, American Association of State Highway and Transportation Officials (AASHTO), 2004.


Stopping Sight Distance (SSD) provides sufficient distance for drivers to anticipate and avoid collisions. However, in some cases this may require a major road vehicle to stop or slow to accommodate the maneuver by a minor road vehicle. To enhance traffic operations, sight distances that exceed the recommended SSD (as shown in Table 3) are desirable. Note that design intersection sight distance criteria for stop-controlled intersections are longer than stopping sight distance to ensure the intersection operates smoothly.

3.3.2. Skewed Geometry

Optimally, an intersection should be designed to have roadways cross at a 90-degree angle. In situations where the intersecting angles are 60 degrees or less, the intersections are considered skewed (see Figure 4).

Figure 4. Skewed Intersection
Aerial photograph of an intersection that is shaped mor like an 'x' than a 't.'

Potential problems associated with skewed intersections include:

When crashes are occurring at skewed intersections, it may be desirable to reduce or eliminate the skew angle of the approaches. Treatments include pavement marking, delineator islands, and roadway realignment.

3.4. Field Reviews

Regardless of implementation approach, a field review should be conducted at identified locations. Intersection field reviews have the potential to identify safety issues and solutions. Field reviews can be conducted as informal field assessments or formal Road Safety Audits (RSAs). An informal field assessment is generally performed by an in-house team with available personnel. The team will spend time at identified intersections and document safety issues to develop recommendations for improvement.

An RSA is a formal safety performance examination of an existing or future road or intersection by an independent, multidisciplinary team. The process includes a formal report on existing or potential road safety issues and identifies opportunities for safety improvements for all road users.10

When conducting field reviews at intersections, one source of information to reference is the MUTCD. It provides the minimum standards for the installation and maintenance of traffic control devices on all public streets, highways, bikeways, and private roads open to public traffic.11 Complying with the MUTCD standards is an important step toward a safer transportation system. If the intersection is not in compliance with the MUTCD, it should be brought up to standard. Non-compliance is an important consideration that can affect road safety and may have liability implications.

There are several elements to consider for this review, as listed below. The appropriate MUTCD sections are noted in parentheses and summary information pertinent to intersections is found in Appendix B. These elements are:

Action: Analyze crash data to determine crash frequency and crash clusters.

Calculate the crash rate of identified locations for comparison and prioritization.

Identify intersections with common safety-related characteristics for potential systematic treatment of safety strategies.

Conduct field reviews of selected locations to determine their compliance with the MUTCD and identify any other potential safety issues and countermeasures.




9 Modified from the American Association of State Highway and Transportation Officials (AASHTO) publication Policy on Geometric Design of Highway and Streets, 5th Edition, 2004. [ Return to note 9. ]

10 More information on RSA's can be found in Appendix A: Resources and References. [ Return to note 10. ]

11 Federal Highway Administration, Manual on Uniform Traffic Control Devices, Washington, DC: December 2009. The MUTCD can be accessed at http://mutcd.fhwa.dot.gov [ Return to note 11. ]


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Page last modified on September 5, 2011.
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