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Home > Intersection Safety > Signalized Intersections > Presentation: Safety at Signalized Intersections

Safety at Signalized Intersections (Long Version)

speaker notes:

This is an approximately 1-hour presentation on the topic of red-light running (RLR). Topics covered include:

• Nature and magnitude of the problem
• Documents available to help with countermeasure selection
• Types of crashes
• Countermeasures and associated crash reduction factors

slide 2

FHWA Safety Focus Areas

speaker notes:

Major Points to Make:

FHWA has identified these 4 primary focus areas for improving safety and reducing crashes. This presentation will look exclusively at safety issues related to SIGNALIZED INTERSECTIONS.

Fatalities attributed to the four primary focus areas:

• Intersections – 21%
• Roadway Departure – 58%
• Pedestrians – 11%
• Speeding – 32%

slide 3

Nationwide Fatalities

There were 41,059 highway fatalities in 2007. Where did they occur?

About half of all crashes and half of all injury crashes occur at intersections.

speaker notes:

Major Points to Make:

This slide represents the most recent published crash data in the FARS database. Includes only FARS elements of "intersection" and "intersection-related" fatalities.

slide 4

Intersection Fatalities

There were 8,657 intersection fatalities in 2007. Where did they occur?

speaker notes:

Major Points to Make:

Looking at just intersection fatalities in general, we see that well over 1/2 occur in urban areas and the majority occur on arterial facilities. 60% of intersection fatalities occur on arterial streets.

• For rural fatalities, about 9% occur at signalized intersections
• For urban fatalities, about 39% occur at signalized intersections

Source: FARS database

slide 5

Traffic Signals

There are at least 3 million intersections in the United States.

At least 300,000 are signalized.

speaker notes:

Major Points to Make:

These are very gross estimates. There is no nation-wide inventory of traffic signals. The point to make is that roughly 10% (but likely even less) of the intersections in the United States are signalized.

slide 6

Intersection Fatalities

speaker notes:

Major Points to Make:

• As noted in the previous slide, roughly 10% of intersections are signalized. However, fatalities at signalized intersections constitute 1/3 of the total number of fatalities.
• The data are from FARS for 2007 and includes only the FARS elements of "intersection" and "intersection-related" fatalities.

Other notes:

• 32% of all intersection fatalities occur at signalized intersections.
• The 5,539 fatalities at UNSIGNALIZED intersections include 3,082 at stop-controlled, 131 at other regulatory control (such as yield), 2,689 where the type of control is either unknown (363) or was not considered a contributor to the fatal crash.
• Crashes at signalized intersections tend to be different than those at unsignalized intersections in both type of crash and severity level.

slide 7

Intersection Safety Guidance

• NCHRP Report 500 Volume 12
• Guide sheets
• Safety Strategies brochure

speaker notes:

Major Points to Make:

• FHWA has numerous resources available to help traffic and transportation engineers analyze intersections from a safety viewpoint.
• Web sites will be shown at the end of the presentation.
• Many are free to download from the Internet.
• Another document that is nearing completion that will help identifying low cost countermeasures is: Low-Cost Countermeasures to Deploy at Stop-Controlled and Signalized Intersections Experiencing Crashes. This document is currently anticipated to be complete and posted on the FHWA Office of Safety web site in the Spring of 2009.

slide 8

Typical Signalized Intersection Crash Types

• Right angle
• Rear end
• Left turn
• Sideswipe
• Pedestrian/bicycle

speaker notes:

Major Points to Make:

• These are the most common crash types that occur at signalized intersections. Each type will be highlighted with typical countermeasures in upcoming slides.
• Right angle crashes are between vehicles on perpendicular approaches.
• Left turn crashes occur between vehicles on opposite approaches where one vehicle is turning left and the opposing vehicle is going straight.

slide 9

Crash Reduction Factors

• Quantitative results from research or other studies
• Expected reduction in crashes from implementation of a specific countermeasure

speaker notes:

Major Points to Make:

• Crash reduction factors (CRFs) provide a quick way for transportation agencies to estimate crash reductions associated with highway safety improvements. Many States and local jurisdictions use these factors to make program-planning decisions about implementing specific treatments and/or to quickly determine the costs and benefits of selected alternatives.
• CRFs are the quantitative results from research and/or evaluation studies, indicating the percentage reductions in crashes that can be expected after implementing treatments or programs. Not all countermeasures presented have CRFs associated with them.
• The countermeasures shown in this presentation are taken primarily from NCHRP Report 500: Volume 12 – A Guide for Reducing Collisions at Signalized Intersections and from the Toolbox Brief (graphic). All of the countermeasures discussed in the report ARE NOT included in this presentation. This is only a sample of recommended countermeasures.
• Not all of the countermeasures have been subjected to studies to determine their CRFs (as evidenced by their categorization as either Proven, Tried or Experimental in the NCHRP Report 500 Volumes 5 & 12)

slide 10

Angle Crashes

• Account for 42% of fatal crashes at signalized intersections
• Potential countermeasures:
  • Optimize change intervals
  • Improve sight distance
  • Restrict access
  • Provide targeted enforcement
  • Restrict parking
  • Construct roundabouts

speaker notes:

Major Points to Make:

• This is primarily a pass-through slide. Point out the type of crash and that angle crashes account for 42% of fatal crashes at signalized intersections. Each of the potential countermeasures will be discussed in the upcoming slides.
• Not all of the countermeasures covered in Report 500 are illustrated in this presentation.
• Angle crashes are between vehicles on perpendicular approaches (as shown in the diagram).
• Left turn crashes occur between vehicles on opposite approaches where one vehicle is turning left and the opposing vehicle is going straight.
• Also of note is that fatal angle crashes involving motorcycles are higher in proportion to all vehicles.
• Angle crashes proportion involving motorcycles is around 70%. 13% of ALL fatal crashes at signalized intersections involve motorcycles.

slide 11

Optimize Change Intervals

speaker notes:

Major Points to Make:

• Too long intervals — right-angle crashes
• Too short intervals — rear-end crashes
• If they are not set properly, optimizing change intervals to the time determined using the ITE formula is a very low cost countermeasure and very effective. It is critical to the success of any potential red-light camera program.
• Research shows that yellow interval duration is a significant factor affecting the frequency of red-light running and that increasing yellow time to meet the needs of traffic can dramatically reduce red light running.
• When yellow intervals are set too short for the prevailing speed, there is likely to be a higher incidence of red-light running due to drivers being caught in the dilemma zone.

slide 12

Improve Sight Distance

speaker notes:

Major Points to Make:

• Sight distance is often overlooked at signalized intersections doe to the more positive control of side street traffic. However, it can have an effect on right-turn-on-red vehicles. In the photo on the slide, the front of the red truck is stopped at a red light and may turn right. However, the truck driver's sight distance is blocked by the overgrown shrub.
• The most difficult aspect of this strategy is the removal of sight restrictions located on private property. The legal authority of highway agencies to deal with such sight obstructions varies widely, and the time (and possibly the cost) to implement sight distance improvements by clearing obstructions may be longer if those obstructions are located on private property.
• This countermeasure can also reduce left turn crashes where the sight obstruction is caused by left-turning vehicles blocking the view of oncoming through traffic.
• Research has shown that by clearing sight triangle of obstructions at UNSIGNALIZED intersections, crashes can be reduced by about 5% PER quadrant cleared.

slide 13

Restrict Access

speaker notes:

Major Points to Make:

• Restricting access to commercial properties near intersections by closing driveways on major streets, moving them to cross streets, or restricting turns into and out of driveways will help reduce conflicts between through and turning traffic. Such conflicts can lead to rear-end and angle crashes related to vehicles turning into and out of driveways and speed changes near the intersection and the driveway(s).
• Locations of driveways on both the cross street and major street should be determined based on the probability that a queue at the signal will block the driveway. Directing vehicles to exits on signalized cross streets will help eliminate or restrict the access to the main roadway. Restricting turns to right-in and right-out only will address conflicts involving vehicles turning left from the road and left from the driveway.
• In this photo, the driveway on the left is near a signalized intersection and has been restricted to right-in/right-out by placing flexible posts in the median. This reduces the chances of right-angle collisions.

slide 14

Provide Targeted Enforcement

speaker notes:

Major points to make:

• Enforcement is a potential countermeasure to unsafe and illegal motorist behavior at intersections. Studies report the reduction of traffic law violations when enforcement is used.
• It is important to correctly identify intersections that would benefit from enforcement. Care should be taken to first ensure that the existing signals are operating properly, are visible, and meet Manual on Uniform Traffic Control Devices requirements, as well as that timing plans—including change intervals—are appropriate.
• Photo on left is a red light camera installation. Photo on the right is a enforcement assistance light that allows officers to be downstream of a traffic signal to watch for red light runners.
• Automated enforcement can also reduce left turn collisions as noted in the table.
• Traditional/conventional enforcement is also part of this countermeasure treatment.

slide 15

Restrict Parking

speaker notes:

Major Points to Make:

• Parking adjacent to turning and/or through lanes on intersection approaches may create a hazard. It can cause a frictional effect on the through traffic stream, can often block the sight triangle of stopped vehicles, and may occasionally cause the blocking of traffic lanes as vehicles move into and out of parking spaces. Restricting and/or eliminating parking on intersection approaches can reduce the workload imposed on the driver and limit additional collision opportunities. Parking restrictions can be implemented through signing, pavement markings, or restrictive channelization. Restrictions can be implemented for specific times of day or specific vehicle types. Enforcement of parking restrictions, accompanied by public information, including towing offending vehicles, is a necessary component of this strategy.
• The Institute of Transportation Engineer's Traffic Engineering Handbook states that, based upon a review of crash data, 20% of non-freeway crashes in cities are in one way or another related to parking. Mid-block crash rates on major streets with parking stalls that are used about 1.0 million hours per year per mile could be expected to decrease up to 75% after parking is prohibited.
• ITE reports a 49% decrease in all crashes when parking is restricted near an intersection.

slide 16

Convert to Roundabout

speaker notes:

Major Points to Make:

• The modern roundabout is a circular intersection with design features that promote safe and efficient traffic flow whereby vehicles travel counterclockwise around a raised center island, with entering traffic yielding the right-of-way to circulating traffic. Drivers approaching a roundabout must reduce speed, be prepared to stop for pedestrians and bicyclists, and look for potential conflicts with vehicles already in the circulatory roadway.
• Appropriate geometric curvature on the roundabout entry controls speeds. Once in the roundabout, vehicles maintain low speeds due to deflection of traffic around the center island as drivers proceed to the appropriate exit following the guidance provided by traffic signs and pavement markings. Modern roundabouts have key differences that distinguish them from older traffic circles and rotaries. Design guidance for roundabouts is provided in Roundabouts: An Informational Guide.

slide 17

Rear End Crashes

• Account for 8% of fatal crashes at signalized intersections
• Potential countermeasures:
  • Increase visibility of intersection and/or traffic signals
  • Increase awareness
  • Improve signal coordination
  • Install turn lanes
  • Control approach speeds
  • Optimize change intervals*

* Discussed previously

speaker notes:

Major Points to Make:

• This is primarily a pass-through slide. Point out the type of crash and that rear-end crashes account for 8% of fatal crashes at signalized intersections. Each of the potential countermeasures will be discussed in the upcoming slides.
• * Discussed previously means that while these countermeasures are applicable to this type of crash, they are also applicable to a previously discussed crash type.

slide 18

Increase Visibility of Signals

speaker notes:

Major Points to Make:

• Lack of visibility of traffic control devices may contribute to crash experience at signalized intersections. Visibility of traffic signals and signs at intersections may be obstructed by physical objects or may be obscured by weather conditions. Also, a driver's attention may be focused on other objects at the intersection, such as extraneous signs. Poor visibility of signs and signals may result in vehicles not being able to stop in time for a signal change or otherwise violating the intended message of a regulatory or directional sign.

Methods for improving visibility of traffic signals and signs include the following:

• installing additional signal head(s) (photo on next slide)
• providing visors to shade signal lenses from sunlight
• providing louvers, visors, or special lenses so drivers are able to view signals only for their approach
• installing backplates (photo on next slide)
• installing larger (12-inch) signal lenses (left photo)
• removing or relocating unnecessary signs
• providing supplemental near-side and/or far-side signal faces (right photo)

slide 19

Increase Visibility of Signals

speaker notes:

Major Points to Make:

• Continuation of previous slide.
• The top photo is an example of providing one signal head for each lane on the approach. By providing a head for each lane, there is less doubt for the driver as to which signal head applies to them. With the use of LED signals, the power consumption is not as considerable as with incandescent signals.
• The bottom/right photo shows a signal in the process of having backplates installed. The target value for the signal at the end of the mast arm is much higher than for the other two signal heads.

slide 20

Increase Awareness of Intersection

speaker notes:

Major Points to Make:

• Some crashes at signalized intersections may occur because drivers are unaware of the presence of an intersection or are unable to see the traffic control device in time to comply.
• The ability of approaching drivers to perceive signalized intersections immediately downstream can be enhanced by:
  • Signing (right photo)
  • Delineation
  • Lighting
  • Warning devices (left photo)
• A technique that has been tried on rural high-speed approaches is "dilemma zone protection." Dilemmas zone protection can assist drivers (esp. heavy vehicle operators) caught in the dilemma zone by extending green times. The photo on the left is a special treatment tied in to dilemma zone protection that alerts drivers of a possible signal change due to a force-off.

slide 21

Improve Signal Coordination

speaker notes:

Major Points to Make:

Signal coordination has long been recognized as having beneficial effects on the quality of traffic flow along a street or arterial. Good signal coordination can also generate measurable safety benefits.

Coordinated signals produce platoons of vehicles that can proceed without stopping at multiple intersections. Reducing the number and frequency of required stops and maintaining constant speeds for all vehicles reduce rear-end conflicts. In addition, signal coordination can improve the operation of turning movements. Drivers may have difficulty making permitted turning maneuvers because of a lack of gaps in through traffic. Crashes may occur when drivers become impatient and accept a gap that is smaller than needed. Such crashes could be reduced if longer gaps were made available.

slide 22

Install Turn Lanes

speaker notes:

Major Points to Make:

• Right-turn lanes can minimize collisions between vehicles turning right and following vehicles, particularly on high-volume and high-speed major roads. A right-turn lane may be appropriate in situations where a high number of rear-end collisions on a particular approach occur.
• Left-turn lanes allow separation of left-turn and through-traffic streams, thus reducing the potential for rear-end collisions. Because they provide a sheltered location for drivers to wait for a gap in opposing traffic, left-turn lanes may encourage drivers to be more selective in choosing a gap to complete the left-turn maneuver.

slide 23

Control Approach Speeds

speaker notes:

Major Points to Make:

• Enforcement of traffic regulations is an important part of an overall intersection safety improvement strategy, but limited resources constrain the efforts police can devote to providing speed enforcement.
• Slowing vehicle speeds on intersection approaches can improve safety for motorists, pedestrians, and bicyclists. Various techniques for attempting to control speeds on approaches involve geometric design, signal control technology, and other traffic calming treatments.
• The top photo shows a speed reduction and advance warning with speed advisory plates in advance of a signalized intersection. This is an example of low cost countermeasure.
• The photo on the bottom is of a photo radar installation (not low cost).

slide 24

Left Turn Crashes

• Account for 21% of fatal crashes at signalized intersections
• Potential countermeasures:
  • Employ protected left turn phasing
  • Implement turn restrictions
  • Improve turning lane design
  • Reconstruct approaches
  • Improve sight distance*
  • Improve signal coordination*

* Discussed previously

speaker notes:

Major Points to Make:

• This is primarily a pass-through slide. Point out the type of crash and that left turn crashes account for 21% of fatal crashes at signalized intersections. Each of the potential countermeasures will be discussed in the upcoming slides.
• * Discussed previously means that while these countermeasures are applicable to this type of crash, they are also applicable to a previously discussed crash type.
• Improving sight distance by removing median obstacles can help reduce left turn crashes (see slide 12).
• Improving signal coordination can reduce left turn crashes by possibly having platoons arrive on an approach to a side street when there are fewer oncoming vehicles (see slide 21).

slide 25

Employ Protected Left Turn Phasing

speaker notes:

Major Points to Make:

• Left turns are widely recognized as the highest-risk movements at signalized intersections. Protected left turn phases significantly improve the safety for left-turn maneuvers by removing conflicts with the left turn.
• A separate phase for the left-turn movement may reduce delay for the vehicles turning left but could result in more overall delay. The length of signal phase and cycle length should be compatible with the left-turn lane length.
• This countermeasure can also reduce sideswipe crashes.
• Note: the range in expected crash reduction factors depends on the level of traffic (ADT) on the street.

slide 26

Implement Turn Restrictions

speaker notes:

Major Points to Make:

• Safety at some signalized intersections can be enhanced by restricting or prohibiting turning maneuvers. This strategy can be applied during certain periods of the day or by prohibiting particular turning movements altogether.
• Prohibition of RTOR can help reduce crashes related to limited sight distance and pedestrians that involve right-turning vehicles. This strategy can also help reduce the frequency and severity of crashes between vehicles turning right on red and vehicles approaching from the left on the cross street or turning left from the opposing approach.
• One key to success is providing for safe and adequate alternative locations to make the turn in close proximity to the intersection where the prohibition is placed.

slide 27

Improve Turn Lane Design

speaker notes:

Major Points to Make:

• The design of the left-turn lane is crucial to its effectiveness as either a safety or operational improvement strategy.
• When providing left-turn lanes, vehicles in opposing left-turn lanes may block the respective driver's view of approaching vehicles in the through lanes. This potential problem can be resolved by offsetting the left-turn lanes.
• There are currently no known CRFs for this treatment.

slide 28

Reconstruct Approaches

speaker notes:

Major Points to Make:

• Signalized intersections may have such a significant crash problem that the only alternative is to change the nature of the intersection itself. Thus, low-cost, short-term solutions will often not be available. Implementing these strategies will necessitate significant public involvement and stakeholder activity.
• Some treatments that can address both safety and operational problems related to left turns are:
  • Reduce/eliminate skewed approaches (left photo)
  • Indirect left-turn treatments, such as jughandles before the crossroad
  • Directional median crossovers
  • Loop roadways beyond the crossroad (right photo)
  • Converting to/from offset-T's
• The photo on the left is an example of realigning an approach to improve safety. The "old" intersection on the right has a significant skew (a well documented safety problem). Therefore, the approach was realigned and a new intersection was created on the left that intersects closer to a 90 degree angle. The old intersection still exists, but services only a small number of "local" vehicles.

slide 29

Sideswipe Crashes

• Account for 13% of fatal crashes at signalized intersections
• Potential countermeasures:
  • Install pavement markings
  • Provide protected left turn phasing*

* Discussed previously

speaker notes:

Major Points to Make:

• This is primarily a pass-through slide. Point out the type of crash and that sideswipe crashes account for 13% of fatal crashes at signalized intersections. Each of the potential countermeasures will be discussed in the upcoming slides.
• Protected left turn phasing can also help reduce sideswipe crashes by reducing the decision workload on the driver.
• * Discussed previously means that while these countermeasures are applicable to this type of crash, they are also applicable to a previously discussed crash type.

slide 30

Install Pavement Markings

speaker notes:

Major Points to Make:

• Providing positive guidance for drivers, especially at complex intersections, can help reduce collisions.

slide 31

Pedestrian/Bicycle Crashes

• Account for 25% of fatal crashes at signalized intersections
• Potential countermeasures:
  • Improve signal hardware
  • Improve pedestrian/bicycle facilities
  • Provide information and education

speaker notes:

Major Points to Make:

• This is primarily a pass-through slide. Point out the type of crash and that pedestrian/bicycle crashes account for 25% of fatal crashes at signalized intersections. Each of the potential countermeasures will be discussed in the upcoming slides.

slide 32

Improve Signal Hardware

speaker notes:

Major Points to Make:

• Nearly one-third of all pedestrian-related crashes occur at or within 50 feet of an intersection (according to NCHRP Report 500 Volume 12). At traffic signals, the biggest crash problem for pedestrians comes from drivers making a permissive left turn across the crosswalk with the walk signal. Thirty percent involve a turning vehicle and another 22% involve a pedestrian either running across the intersection or darting in front of a vehicle whose view was blocked just prior to impact. Another 16% of these intersection-related crashes occur because of driver violation (e.g., failure to yield the right-of-way). (also according to NCHRP Report 500 Volume 12)

Traffic control hardware improvements that can be made to an intersection to increase pedestrian safety include the following:

• pedestrian signs, signals (including countdown signals), and markings
• pedestrian-only phase or pedestrian-lead phase during signal operation
• technology to show a push button is working

slide 33

Improve Pedestrian/Bicycle Facilities

speaker notes:

Major Points to Make:

• The mix of travel modes at intersections, along with the vehicle-vehicle conflicts possible, can create safety and operational concerns for non-motorists.
• Other traffic control improvements that can be made to an intersection to increase pedestrian and bicycle safety include the following:
  • pedestrian signs and markings
  • bicycle lanes markings through the intersection
  • lights in crosswalks in school zones
  • prohibition of right turn on red (RTOR)
  • public information or signs that educate pedestrians regarding use of push buttons
• The CRF for Provide Sidewalks (both sides) is 50-90% of crashes involving pedestrians walking along the roadway.
• The CRF of 36% for bicycle lanes is from a Danish study.

slide 34

Information and Education

speaker notes:

Major Points to Make:

• Information and education can be geared towards reducing crashes of ALL TYPES. However, it is presented here because of the fairly easy way to target pedestrians and bicyclists in schools and other civic organizations.
• Providing targeted public information and education (PI&E) on safety problems at intersections is a preventive measure that can help improve driver compliance with traffic control devices and traffic laws. PI&E programs generally add effectiveness to targeted enforcement programs as well.
• Another option is to develop public information campaigns aimed at specific drivers who violate regulations at intersections, even though it is often difficult to identify and focus upon a subset of the driving population using a specific intersection. Therefore, an area-wide program is often the preferred approach.
• Data on the effectiveness of this strategy for this specific application are not known, but it is expected that providing information to drivers will help improve safety at intersections. It may not be possible to identify or reach the entire audience that would benefit from a PI&E campaign.

slide 35

For More Information

NCHRP Report 500 Series Volume 12
http://safety.transportation.org/

Manual on Uniform Traffic Control Devices
http://mutcd.fhwa.dot.gov/

Signalized Intersections: Informational Guide
http://www.fhwa.dot.gov/publications/research/safety/04091/

FHWA Office of Safety
http://safety.fhwa.dot.gov/

speaker notes:

Major Points to Make:

• These web sites can be accessed for further information on the material contained in this presentation.

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Program Contact

Jeffrey Shaw
jeffrey.shaw@dot.gov
708-283-3524

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