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Publication Number: FHWA-RD-02-089
Date: July 2002

Safety Effectiveness of Intersection Left- and Right-Turn Lanes

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APPENDIX A. SUMMARY OF INTERSECTION SAFETY STUDIES

The findings of the literature review are presented in this extensive summary table, table A-1. The table presents the following information about each source identified in the literature:

  • General topic (i.e., geometric design or traffic control element).
  • Author, publication year, and reference number.
  • Summary of major findings.
  • Study type (e.g., before/after, comparative, predictive model).
  • Data used in any analyses that were conducted (including the number of sites, if available).
  • Type of sites.

The reference numbers in the table provide a link to the reference list presented at the end of this report. The summary of major findings describes the nature of the relationship between particular geometric design, traffic control, or traffic volume factors and safety indicated by each study. Where the safety relationship in a particular reference can be expressed as a simple percentage difference or algebraic difference, that quantitative value is presented in the major findings column. However, where the findings are more complex, such as the results of predictive modeling or classification and regression tree (CART) analysis, they could not always be presented quantitatively in the table.

Design and control elements Related literature Major findings Study type Data used Type of site
Intersection Geometric Design Features
Left-turn lanes Harwood et al. [2000](25) • Based upon the judgement of an expert panel, installation of a left-turn lane along one major approach reduces intersection-related accidents by 18 to 24%, depending upon the type of traffic control and number of legs at the intersection.

• Based upon the judgement of an expert panel, installation of left-turn lanes along both major approaches to a four-leg intersection reduces intersection-related accidents by 33 to 42 percent, depending upon the type of traffic control.

Accident prediction algorithm using negative binomial distribution and accident modification factors developed by expert panel. Prediction algorithm combines elements of historical accident data, predictions from statistical models, results of before-after studies, and expert judgments made by experienced engineers. Rural intersections along two-lane highways.
Vogt [1999](24) For a four-lane by two-lane STOP-controlled rural intersection, the predictive model indicates installation of left-turn lanes along the major approach reduced total accidents by 38 percent. Accident prediction model using negative binomial distribution. 72 four-leg intersections in California and Michigan. Four-lane by two-lane STOP-controlled rural intersections.
Gluck et al. [1999](6) Installation of left-turn lanes reduced the accident rates per million entering vehicles at unsignalized intersections by 50 percent and at signalized intersections by 18 percent. Combined the presence of left-turn lanes reduced the accident rate by 35 percent. Synthesis of previous research conducted by Tamburri and Hammer(13) and Wilson et al.(14) 53 intersections in California.  
Gluck et al. [1999](6) Intersections without turn lanes had an accident rate of 1.65 accidents per million entering vehicles, while intersections with left-turn lanes had an accident rate of 0.59. Synthesis of previous research conducted by Shaw and Michael.(65) Eight intersections without lanes; three with left-turn lanes.  
Gluck et al. [1999](6) Installation of left-turn lanes reduced the accident rate per million entering vehicles by 38 percent. Synthesis of previous research conducted by Ben-Yakov and Craus(11) and Craus and Mahalel.(12) 25 intersections.  
Left-turn lanes (continued) Gluck et al. [1999](6) Installation of left-turn lanes reduced the accident rates per million left-turning vehicles at unsignalized intersections by 77 percent and at signalized intersections by 54 percent. Synthesis of previous research conducted by Agent.(10)    
Gluck et al. [1999](6) Restriping the lane assignments to provide left-turn lanes reduced the number of accidents at eight intersection locations. The left-turn lanes reduced left-turn accidents by 62 percent and all accidents by 58 percent. Synthesis of previous research conducted by Greiwe.(9) Eight intersections in Indiana.  
Gluck et al. [1999](6) 1.8-mile section of four-lane roadway was converted to three-lane cross-section. Total number of accidents before conversion was 109, and 67 accidents occurred during after period. Synthesis of previous research conducted by New Jersey Department of Transportation.(7) 1.8 miles of Route 47 in New Jersey converted from four-lane road to three-lane road.  
Gluck et al. [1999](6) • Installation of left-turn lanes along eight-mile southern section of Route 130 reduced the accident rate per million entering vehicles by 35 percent.

• Installation of left-turn lanes along 28-mile northern section of Route 130 reduced the accident rate per million entering vehicles by 51 percent.

Synthesis of previous research conducted by New Jersey Department of Transportation.(8) Eight mile southern section of Route 130 in New Jersey and 28-mile northern section.  
Bauer and Harwood [1996](20) Left-turn channelization resulted in an increase in total multiple-vehicle accidents and fatal injury accidents. Statistical modeling with negative binomial regression. 14,432 rural intersections in California. Rural and urban signalized and unsignalized intersections.
Poch and Mannering [1995](22) Total accident frequencies were found to be higher on intersection approaches with a shared through-left lane and two or more total lanes than on approaches with other conditions. Approaches with a left-turn lane, a through lane, and a shared through-right lane had more rear-end accidents than those with other conditions. Accident prediction model using negative binomial distribution. 63 intersections in Bellevue, Washington. Urban areas. A large number of intersections were in residential areas which are characterized by low traffic volume. All intersections had some sort of operational improvement during 1988-92.
Left-turn lanes (continued) Maze et al. [1994](23) Predictive models indicate that a left-turn lane with permitted phasing at a signalized intersection has a positive effect on safety. A typical example developed by the authors indicates an anticipated reduction in left-turn accident rate of approximately 5.5 percent from installation of a left-turn lane with permitted phasing. Statistical modeling based on multiple regression. 63 signalized intersections, including 248 intersection approaches. Five years of accident data were considered for each intersection. At-grade signalized intersections in Iowa.
Maze et al. [1994](23) Predictive models indicate that a left-turn lane with protected/permitted phasing at a signalized intersection has a positive effect on safety. A typical example developed by the authors indicates an anticipated reduction in left-turn accident rate of approximately 35 percent from installation of a left-turn lane with protected/permitted phasing. Statistical modeling based on multiple regression. 63 signalized intersections, including 248 intersection approaches. Five years of accident data were considered for each intersection. At-grade signalized intersections in Iowa.
McCoy and Malone [1989](4) On urban four-lane roadways, left-turn lanes at signalized and unsignalized intersections significantly reduced rear-end, sideswipe, and left-turn accidents. At unsignalized intersections with left-turn lanes, there was also a significant increase in right-angle accidents. Comparative. 63 intersections on urban, four-lane roadways. Urban signalized and unsignalized intersections in Nebraska.
Lau and May [1988](40) Left-turn channelization on the crossroad was found to be a significant factor in predicting injury accidents at unsignalized intersections. CART analysis of residuals from base model. 17,000 unsignalized intersections. Seven years of injury accident data for each intersection. Unsignalized intersections on California state highways.
Left-turn lanes (continued) Hauer [1988](15) • Provision of left-turn channelization at unsignalized intersections reduced accidents by 70 percent in urban areas when combined with curbs or raised bars. Likewise, accidents were reduced by 65 and 60 percent, respectively, in suburban and rural areas.

• When channelization was painted at unsignalized intersections, accidents decreased by 15, 30, and 50 percent in urban, suburban, and rural areas, respectively.

Synthesis of previous research conducted by McFarland et al. [1979].(16) Not available. Not available.
Hauer [1988](15) At signalized intersections, left-turn channelization with a left-turn phase reduced accidents by 36 percent and without the left-turn phase by 15 percent. Synthesis of previous research conducted by McFarland et al. [1979].(16) Not available. Not available.
Hauer [1988](15) Adding left-turn lanes reduced accidents by varying amounts depending on the type of intersection, whether it was signalized or unsignalized, and whether the intersection was rural or urban. Synthesis of previous research conducted by R. Jorgensen and Associates, Inc. [1978].(66) Not available. Not available.
McCoy et al. [1985](21) At unsignalized intersections on rural two-lane highways, there was no significant difference in rear-end and left-turn accident rates between intersections with left-turn lanes and those without left-turn lanes. Comparative. Intersections on rural two-lane highways in Nebraska. Unsignalized intersections.
Parker et al. [1983](3) Passing-related accidents at rural intersections along two-lane highways do not represent a major safety problem, but when a left-turn lane is provided at new or reconstructed intersections, potential for passing-related accidents is greatly reduced. Benefit/cost analysis not reviewed directly. Overview based on synthesis of previous research by Kuciemba and Cirillo [1992].(49) Not available. Not available.
Left-turn lanes (continued) David and Norman [1976](26) Signalized intersections with opposing left-turn lanes were found to have significantly more accidents than intersections without opposing left-turn lanes. Provision of opposing left-turn lanes at four-leg signalized intersections was found to increase accident frequencies by 2.4 to 6.1 accidents per year. Comparative. 22 four-leg intersections with opposing left-turn lanes; it is not clear how many four-leg intersections without left-turn lanes were available. Three years of accident data were obtained for each intersection. Urban intersections in the San Francisco Bay Area of California.
Dale [1973](19) At intersections along rural two-lane highways, installation of a traffic signal and left-turn lane reduced the total number of accidents by 19.7 percent, while the installation of a traffic signal without left-turn channelization reduced the total number of accidents by 6 percent.      
Foody and Richardson [1973](5) • For signalized intersections, the accident rate was reduced 38 percent with the addition of left-turn lanes.

• For unsignalized intersections, the accident rate was reduced 76 percent with the addition of left-turn lanes.

Comparative. Not available. Not available.
Lacy [1972](18) Several improvements to the intersection, which included extending and rearranging the channelization and adding separate left-turn lanes, reduced the accident frequency by 35 percent and the accident severity by 80 percent. Additional improvements to the intersection included widening the approaches and modifying the traffic signals. Before/After. One urban intersection in Peoria, Illinois. Urban intersection.
Caltrans [1967](17) Reduction in accident rates at unsignalized intersections was much higher with use of raised barrier left-turn lanes than with painted left-turn lanes. Before/After. 53 safety improvement projects in California. Urban and rural areas, including signalized and unsignalized intersections.
Offset left-turn lanes Harwood et al. [1995](27) A field review of traffic operations and an office review of three years of accident data were conducted for two signalized intersections with tapered offset left-turn lanes and one signalized intersection with parallel offset left-turn lanes. This review found no operational or accident problems at the intersections related to the offset left-turn lanes. However, no measures of effectiveness comparing offset and conventional left-turn lanes were developed. Operational and safety review of intersection performance. Field observation of undesirable driver behavior and three years of accident data for three intersections. Signalized intersections on divided highways with tapered and parallel offset left-turn lanes.
McCoy et al. [1992](28) Developed guidelines concerning the amount of offset between opposing left-turn lanes to provide adequate sight distance, but performed no accident studies. Engineering analysis. Typical intersection geometrics. Intersections with opposing left-turn vehicles.
Joshua and Saka [1992](29) Developed guidelines concerning the amount of offset between opposing left-turn lanes to provide adequate sight distance, but performed no accident studies. Engineering analysis. Typical intersection geometrics. Intersections with opposing left-turn vehicles.
Right-turn lanes Harwood et al. [2000](25) • Based on the judgement of an expert panel, presence of a right-turn lane along one major approach to a rural STOP-controlled intersection reduces intersection-related accidents by 5 percent.

• Based on the judgement of an expert panel, presence of right-turn lanes along both major approaches to a rural STOP-controlled intersection reduces intersection-related accidents by 10 percent.

Accident prediction algorithm using negative binomial distribution and accident modification factors developed by expert panel. Prediction algorithm combines elements of historical accident data, predictions from statistical models, results of before-after studies, and expert judgments made by experienced engineers. Rural intersections along two-lane highways.
Right-turn lanes (continued) Harwood et al. [2000](25) • Based on the judgement of an expert panel, presence of a right-turn lane along one major approach to a rural signalized intersection reduces intersection-related accidents by 2.5 percent.

• Based on the judgment of an expert panel, presence of right-turn lanes along both major approaches to a rural signalized intersection reduces intersection-related accidents by 5 percent.

Accident prediction algorithm using negative binomial distribution and accident modification factors developed by expert panel. Prediction algorithm combines elements of historical accident data, predictions from statistical models, results of before-after studies, and expert judgments made by experienced engineers. Rural intersections along two-lane highways.
Vogt and Bared [1998](30) Presence of right-turn lanes at three-leg rural unsignalized intersections increases the total number of intersection-related accidents by 27 percent. Poisson and negative binomial modeling. 389 rural three-leg intersections in Minnesota. Unsignalized intersections on rural two-lane highways.
Bauer and Harwood [1996](20) Right-turn channelization resulted in an increase in total multiple-vehicle accidents and fatal injury accidents. Statistical modeling with negative binomial regression. 14,432 rural intersections in California. Rural and urban signalized and unsignalized intersections.
Channelization Hauer [1988](15) Channelization was found to reduce accidents by 32 percent and injury accidents by 50 percent. The average benefit-cost ratio of channelization was 2.3. Synthesis of previous research conducted by Hagenaur et al. [1982](67) Not available. Not available.
David and Norman [1976](26) Raised pavement markings tended to decrease accidents, especially at cross intersections. Comparative. 558 intersections with 4,372 accidents in three years in the San Francisco Bay Area of California. Urban areas. 82 percent of intersections had some form of delineation.
Channelization (continued) Exnicios [1967](33) Several safety measures, including re-channelization, were implemented at three intersections. The improvements resulted in a 31 percent reduction in total accidents (over two years), a 58 percent reduction in total accidents (over one year), and a 100 percent reduction in total accidents (over 26 months) at the respective intersections. Before/After. Three intersections. The intersections were located in a residential suburb in Chicago, in metropolitan New Orleans, and in Shreveport, Louisiana.
Rowan and Williams [1966](34) Accident rates, personal injuries, and rear-end type accidents were reduced due to the introduction of channelization. Before/After. US Route 290 in northwest Houston. Arterial, four-lane, at signalized intersections.

Channelization

• Island design

Forrestel [1994](37) The pedestrian accident rate at an unsignalized intersection on a four-lane arterial was reduced by 11.5 percent when raised median islands were installed. Synthesis of previous research efforts. Related article entitled "A Comparison of the Pedestrian Safety of Median Islands and Marked Crossings." [1978](49) One intersection in Western Australia. Four-lane, unsignalized intersection.
Washington et al. [1990](36) Intersection approaches with raised medians have a 40 percent lower accident rate than those with flush medians. Comparative. 40 intersections in California. Not available.
Templer [1980](32) Raised medians reduced the number of conflicts between pedestrians and vehicles. However, the difference was not statistically significant. Before/After. Two intersections in Clearwater, Florida. Signalized T-intersections, located in business and recreational areas.
Number of intersection legs (e.g., three, four, five) Bauer and Harwood [1996](20) Rural four-leg STOP-controlled intersections have about twice as many accidents as rural three-leg STOP-controlled intersections (1.1 vs. 0.6 accidents per intersection per year). A similar pattern was found for urban STOP-controlled intersections (2.2 accidents per intersection per year for four-leg intersections vs. 1.3 for three-leg intersections). Comparative. 8,525 at-grade intersections in California. Rural four-leg, rural three-leg, urban four-leg, and urban three-leg intersections, all with STOP control.
Number of intersection legs (e.g., three, four, five) (continued) Harwood et al. [1995](27) Predictive relationships were developed for number of multiple-vehicle intersection accidents per year as a function of major-road ADT, crossroad ADT, major-road median width, major-road lane and shoulder widths, major-road design speed, presence of left-turn lanes, and terrain. Results show that typical divided highway intersections with four legs have about twice as many accidents as three-leg intersections for narrow medians and more than five times as many accidents as three-leg intersections for wide medians. Statistical modeling with Poisson regression. 1,200 intersections on California state highways. Urban/suburban unsignalized intersections on divided highways in California.
Hanna et al. [1976](38) In rural areas, four-leg intersections have higher accident rates than T intersections (69 percent increase). Comparative. 232 intersections in rural municipalities in Virginia. Includes both STOP-controlled and signalized intersections.
David and Norman [1976](26) In urban areas at STOP-controlled intersections, accident frequencies were very similar for four-leg intersections and T/Y-type intersections with ADT under 20,000 veh/day. Once above 20,000 veh/day, the accidents doubled for four-leg intersections. Comparative. 558 intersections with 4,372 accidents in three years in the San Francisco Bay Area of California. Of 558 intersections, 269 were three-leg intersections, and 289 were four-leg intersections.
Intersection type (e.g., cross, T, Y, offset) Lau and May [1988](39) Intersection type was found to be a significant factor in predicting injury accidents at signalized intersections. CART analysis of residuals from base model. 2,488 signalized intersections. seven years of injury accident data for each intersection. Signalized intersections on California state highways.
Lau and May [1988](40) Intersection type was found to be a significant factor in predicting injury accidents at unsignalized intersections. CART analysis of residuals from base model. 17,000 unsignalized intersections. Seven years of injury accident data for each intersection. Unsignalized intersections on California state highways.
Intersection type (e.g., cross, T, Y, offset) (continued) Hanna et al. [1976](38) For three-leg intersections, Y intersections were found to have accident rates approximately 50 percent higher than T intersections. Comparative. 232 intersections in rural municipalities in Virginia. Includes both STOP-controlled and signalized intersections.
Hanna et al. [1976](38) For four-leg intersections, offset intersections had accident rates that were approximately 43 percent of the accident rate of conventional four-leg intersections. Comparative. 232 intersections in rural municipalities in Virginia. Includes both STOP-controlled and signalized intersections.
Angle of intersection (e.g., skew) Harwood et al. [2000](25) AMFs for intersection skew angle were derived from statistical modeling and apply to total intersection-related accidents. For a three-leg STOP-controlled intersection, the AMF was calculated as:

AMF = exp (0.0040 SKEW)

For a four-leg STOP-controlled intersection, the AMF was calculated as:

AMF = exp (0.0054 SKEW)

where:

SKEW = intersection skew angle (degrees), expressed as the absolute value of the difference between 90 degrees and the actual intersection angle.

Accident prediction algorithm using negative binomial distribution and accident modification factors developed by expert panel. Prediction algorithm combines elements of historical accident data, predictions from statistical models, results of before-after studies, and expert judgments made by experienced engineers. Rural intersections along two-lane highways.
Bauer and Harwood [1996](20) Angle of intersection was found to have a statistically significant relationship to multiple-vehicle accident frequency at urban, four-leg, signalized intersections, but the direction of the effect was opposite to that expected. Skewed intersections were found to have accident frequencies approximately 20 percent less than 90 intersections. This finding may represent a surrogate effect of some uncontrolled variable. Statistical modeling with negative binomial regression. 198 intersections on California state highways. Urban four-leg signalized intersections in California.
Angle of intersection (e.g., skew) (continued) McCoy et al. [1994](42) At two-way STOP-controlled intersections on rural two-lane highways, the number of accidents per year increases with traffic volume and skew angle. Thus, more accidents will occur with higher volumes and/or greater skew angles. Three-leg intersections have fewer accidents than four-leg intersections with equivalent traffic conditions and skew angles. Comparative. 29 skewed and 39 nonskewed rural intersections in Nebraska. Two-way STOP-controlled intersections on rural two-lane highways. Included three-leg and four-leg intersections. Volumes on the major and minor roadways ranged from 400 to 5,200 veh/day and from 150 to 1,500 veh/day, respectively.
Hauer [1988](15) Stated as an important safety factor. No safety studies conducted.      
Roundabouts See section of this table on type of traffic control.        
Curb return radius Hauer [1988](15) Stated as an important safety factor. No safety studies conducted.      
Sight distance

• Intersection sight distance (clear sight triangles in intersection quadrants)

Harwood et al. [2000](25) Based upon the judgement of an expert panel, the AMFs are as follows for intersection sight distance at intersections with STOP control on the minor leg(s):

• 1.05 if sight distance is limited in one quadrant of the intersection.

• 1.10 if sight distance is limited in two quadrants of the intersection.

• 1.15 if sight distance is limited in three quadrants of the intersection.

• 1.20 if sight distance is limited in four quadrants of the intersection.

Sight distance in a quadrant is considered limited if the available sight distance is less than the sight distance specified by AASHTO policy for a design speed of 20 km/h less than the major road-design speed and the sight distance restrictions are due to roadway alignment and/or terrain.

Accident prediction algorithm using negative binomial distribution and accident modification factors developed by expert panel. Prediction algorithm combines elements of historical accident data, predictions from statistical models, results of before-after studies, and expert judgments made by experienced engineers. Rural intersections along two-lane highways.
Sight distance

• Intersection sight distance (clear sight triangles in intersection quadrants) (continued)

David and Norman [1976](26) Developed estimates of the reduction in annual accident frequency from improving sight distance as a function of the initial sight distance (termed "sight radius" in the study) from the minor-road approach and total entering ADT. The results indicated that, in most cases, the worse the initial sight distance, the greater the accident reduction obtained from a sight distance improvement. Magnitudes of the sight distance improvements were not specified. Comparative. 558 intersections with 4,372 accidents in three years in the San Francisco Bay Area of California. Urban areas where foliage and buildings obstructed the view of intersections.
Hanna et al. [1976](38) In this study, the average accident rate for all intersections was 1.13, while the average accident rate for intersections with "poor sight distance" is 1.33 accidents per million entering vehicles. Comparative. Examined 41 intersections in rural area of Virginia with total of 366 accidents. Rural municipalities, including both STOP-controlled and signalized intersections.
Mitchell [1972](43) • Total accidents at intersections dropped 67 percent when intersection sight obstructions were removed.

• The greatest percentage of reduction in accidents was experienced at the intersections where the sight distance was improved.

Before/After. Five intersections in Concord, California. Sight distance at five intersections that had been improved.
Sight distance

• Stopping sight distance

Fambro et al. [1989](44) Accident rates were high for intersections located on crest vertical curves with limited sight distance. Similar results were obtained in NCHRP Project 3-42 [1996].(28) Comparative.   Rural two-lane roadways.
Sight distance

• Sight distance to traffic control device (e.g., STOP sign, signal)

None found        
Approach width Bauer and Harwood [1996](20) It was found that as lane width decreases, the total number of multiple-vehicle accidents and fatal injury accidents increases. Statistical modeling with negative binomial regression. 2,999 intersections in California. Urban four-leg signalized and unsignalized intersections.
David and Norman [1976](26) Higher accident occurrence for narrow streets was not evident. Comparative. 558 intersections with 4,372 accidents in three years in the San Francisco Bay Area of California. 269 T-intersections, 289 four-leg intersections. 298 of the intersections were STOP-controlled.
Lacy [1972](18) Several improvements to the intersection, which included widening the approaches, reduced the accident frequency by 35 percent and the accident severity by 80 percent. Other improvements to the intersection included: extending and rearranging the channelization, adding separate left-turn lanes, and modifying the traffic signals. Before/After. One urban intersection in Peoria, Illinois. Urban intersection.
Number of approach lanes Bauer and Harwood [1996](20) In the models for total multiple-vehicle accidents and fatal injury accidents at rural and urban unsignalized intersections, approaches with one lane were associated with higher accident frequencies, while approaches with two or more lanes were associated with lower total accident frequencies. The opposite appears to be the case for urban, four-leg, signalized intersections. Statistical modeling with negative binomial regression. 2,262 rural intersections in California. Rural four-leg STOP-controlled intersections.
Lau and May [1988](39) The number of lanes on the major roadway and the crossroad were found to be a significant factor in predicting injury accidents at signalized intersections. CART analysis of residuals from base model. 2,488 signalized intersections. Seven years of injury accident data for each intersection. Signalized intersections on California state highways.
Number of approach lanes (continued) Lau and May [1988](40) The number of lanes on the major roadway was found to be a significant factor in predicting injury accidents at unsignalized intersections. CART analysis of residuals from base model. 17,000 unsignalized intersections. Seven years of injury accident data for each intersection. Unsignalized intersections on California state highways.
David and Norman [1976](26) For roadways with ADT under 10,000 veh/day, accident frequencies can be reduced by providing through lanes. Comparative. 558 intersections with 4,372 accidents in three years in the San Francisco Bay Area of California. Of 558 intersections, 71 percent had 2x2 through lanes, 26 percent had 2x4, and 3 percent had 4x4 on the crossroad and major road, respectively.
Median width and type Harwood et al. [1995](27) • At rural four-leg unsignalized intersections, accident frequency decreases as median width increases.

• At urban/suburban intersections (unsignalized and signalized), accident frequency increases with increasing median width.

Comparative study and statistical modeling with Poisson regression. 2,140 divided highway intersections in urban and rural areas of California. Intersections include rural and urban/suburban unsignalized intersections (four-leg and three-leg), as well as urban/suburban four-leg signalized intersections.
Van Maren [1977](47) Found no statistically significant relationship between median width and intersection accident rate.      
Priest [1964](46) Except at very low volume levels, intersection accident frequencies decrease as the median width increases. The difference in intersection accident rate between medians less than 20 ft wide and medians 20 to 30 ft wide is greater than the difference in intersection accident rate between medians with widths of 20 to 39 ft and those of 40 ft or more. Statistical modeling with regression analysis. 316 intersections in Ohio. Three years of accident data were available for each intersection. Intersections on divided highways with partial or no access control in Ohio.
Vertical alignment on intersection approaches Hanna et al. [1976](38) Rural intersections with steep grades (greater than 5 percent) "generally operate safely." These intersections had an accident rate of 0.97 accidents per million entering vehicles, compared to an overall accident rate of 1.13. Comparative. 232 intersections in Virginia. Rural areas.
Horizontal alignment on intersection approaches None found        
Design speed Bauer and Harwood [1996](20) As design speed decreases, there is an increase in total multiple-vehicle accidents and fatal injury accidents. Statistical modeling with negative binomial regression. 1,434 rural intersections in California. Rural and urban intersections.
Traffic Control and Operational Features
Type of traffic control

• Uncontrolled

• YIELD control

Poch and Mannering [1995](22) With no control on an intersection approach, total and angle accidents decrease. Statistical modeling with negative binomial regression. 63 intersections in the city of Bellevue, Washington. Urban areas. A large number of intersections are in residential areas which are characterized by low traffic volume. All intersections had some sort of operational improvement during 1988-92.
Hauer [1988](15) • 44 percent and 52 percent fewer accidents after conversion to YIELD-control.

• Another study gives accident reduction of 23 and 63 percent after conversion of uncontrolled intersections to YIELD-control.

Synthesis of previous studies. Not available. Not available.
Lau and May [1988](39) Traffic control type was found to be a significant factor in predicting injury accidents at unsignalized intersections. However, the traffic control type variable was formulated in such a way that it is not easy to distinguish among the traffic control types used in this table. CART analysis of residuals from base model. 17,000 unsignalized intersections. Seven years of injury accident data for each intersection. Unsignalized intersections on California state highways.
Hall et al. [1978](50) Accidents can be reduced by 20 to 60 percent through proper use of YIELD signs at low-volume intersections. Little additional reduction is obtained if YIELD signs are replaced by STOP signs. Not available. Not available. Not available.
Agent and Deen [1975](51) At YIELD signs, over half of the accidents were rear-end collisions, while angle collisions made up over half the accidents at STOP signs. Comparative. Data for intersections in Kentucky. Three years of accident data were available. Intersections in rural areas.
Type of traffic control

• STOP control

Hanna et al. [1976](38) Accident rates at STOP-controlled intersections were lower at those intersections having high traffic flow. Comparative. 232 intersections in Virginia. Rural areas.
Type of traffic control

• STOP control with flashing beacons

None found        
Type of traffic control

• Signal control (phasing, timing, and operation)

Poch and Mannering [1995](22) With signal control intersections, the total and angle accidents decrease. Statistical modeling with negative binomial distribution. 63 intersections in city of Bellevue, Washington. Urban areas. All intersections had some sort of operational improvement during 1988-92.
Maze et al. [1994](23) Predictive models indicate that a protected left-turn signal phase without a left-turn lane has a positive effect on safety. A typical example developed by the authors indicates an anticipated reduction in left-turn accident rate of approximately 50 percent from installation of a protected left-turn signal phase. Statistical modeling based on multiple regression. 63 signalized intersections, including 248 intersection approaches. Five years of accident data were considered for each intersection. At-grade signalized intersections in Iowa.
Lau and May [1988](39) A control-type variable based on signal phasing and actuation was found to be a significant factor in predicting injury accidents at signalized intersections. However, the control type variable was defined in such a way that explicit effects of phasing and actuation cannot be determined. CART analysis of residuals from base model. 17,000 unsignalized intersections. Seven years of injury accident data for each intersection. Unsignalized intersections on California state highways.
Hanna et al. [1976](38) Installation of traffic signal controls could result in slight increase in accident rates, significant increase in rear-end accidents, and comparable decreases in angle collisions. Comparative. 232 intersections in Virginia. Rural municipality area.
Type of traffic control

• Signal control (phasing, timing, and operation) (continued)

David and Norman [1976](26) In urban areas, multi-phase traffic signals appear to have lower percentages of fatal and injury accidents than two-phase signals. Comparative. 558 intersections with 4,372 accidents in 3 years in the San Francisco Bay Area of California. Of 558 intersections, 269 were T intersections and 289 were four-leg intersections. 298 of the intersections were STOP-controlled.
King and Goldblatt [1975](52) • Signalization leads to a reduction in right-angle accidents and an increase in rear-end accidents.

• Signalized intersections have higher accident rates, but this is usually offset by less severity per accident.

Comparative analyses and review of related research. Used a large nationwide accident data base. Not available.
Type of traffic control

• Roundabouts

Persaud et al. [2001](53) Converting intersections with conventional traffic control (i.e.,STOP, signal) to roundabouts reduces all accidents by 40 percent, injury accidents by 80 percent, and fatal and incapacitating injury accidents by 90 percent. Before/After. 23 intersections located in 7 states. Mix of urban, suburban, and rural environments.
  Robinson et al. [2000](41) Converting intersections with conventional traffic control (i.e.,STOP, signal) to roundabouts reduces all accidents by 37 percent and injury accidents by 51 percent. Synthesis of research conducted in the U.S. and internationally. Not available. Not available.
Turn prohibitions Lau and May [1988](39) Left-turn prohibitions were found to be a significant factor in predicting injury accidents at signalized intersections. CART analysis of residuals from base model. 2,488 signalized intersections. Seven years of injury accident data for each intersection. Signalized intersections on California state highways.
  Lau and May [1988](40) Left-turn prohibitions were found to be a significant factor in predicting injury accidents at unsignalized intersections. CART analysis of residuals from base model. 17,000 unsignalized intersections. Seven years of injury accident data for each intersection. Unsignalized intersections on California state highways.
Presence and type of crosswalks Hauer [1988](15) Marked crosswalks had more exposure, but fewer accidents, than unmarked crosswalks. Synthesis of previous research conducted by Knoblauch et al. [1984](68) Not available. Not available.
Hauer [1988](15) Pedestrian accidents increased 86 percent after crosswalks were marked, and rear-end collisions increased 32 percent. Synthesis of previous research entitled "What Not To Expect from Crosswalk Signals." [1976](69) Not available. Not available.
Hauer [1988](15) Pedestrian accidents may be reduced by approximately 50 percent by marking crosswalks. Synthesis of previous research conducted by Untermann. [1984](70) Not available. Not available.
Hauer [1988](15) Painted crosswalks reduced violation of the pedestrian's right of way. Synthesis of previous research cited in Traffic Engineering Handbook. [1965](71) Not available. Not available.
  Smith and Knoblauch [1987](53) Accident analyses suggest use of crosswalks at all signalized intersections. Comparative. Not available. Intersections with marked and unmarked crosswalks.
  Herms [1970](55) More pedestrian accidents occurred in marked crosswalks than in unmarked crosswalks by a ratio of about 6 to 1. A crosswalk usage count showed the crosswalk use ratio was approximately 3 to 1, marked vs. unmarked. In terms of usage, approximately twice as many pedestrian accidents occurred in marked crosswalks as compared to unmarked crosswalks. Comparative. 400 unsignalized intersections in San Diego, California. Each intersection had one marked and one unmarked crosswalk crossing the major flow of traffic.
Posted speed limits on approaches None found.        
Advance warning signs Gattis and Iqbal [1994](56) The "Do Not Block the Intersection" sign was found to be ineffective in preventing drivers from blocking intersections. Before/After. Four intersections. Two street intersections and two commercial driveway intersections.
Klugman et al. [1992](58) Accident rates for intersections equipped with advance warning signs with flashers (AWFs) decreased from 1.22 to 1.09 accidents per million entering vehicles for all accident types and decreased from 0.68 to 0.63 for right-angle and rear-end accidents. Comparative and Before/After Study. 14 intersections. Signalized intersections.
Pant and Huang [1992](57) • On tangent approaches to intersections, the Prepare to Stop When Flashing (PTSWF) sign showed no significant increase or decrease in conflict rate.

• On curved approaches to intersections, the PTSWF sign increased the conflict rate by at least 15 percent.

• The Flashing Symbolic Signal Ahead (FSSA) sign showed no effect on vehicle conflict rates.

Before/After. Not available. High-speed signalized intersections on rural or suburban highways in Ohio.
Washington et al. [1991](36) Implementation of AWFs can reduce approach accident rates at high-speed isolated signalized intersections by as much as 50 percent. Comparative. 40 signalized intersections in California. High-speed isolated signalized intersections.
  Washington et al. [1991](36) • Right-angle accidents were significantly reduced with the presence of route markers and/or advance warning signs.

• Accident rates increased on horizontal approaches with a skew that contained advance warning signs.

Synthesis of prior research conducted by Van Maren et al. [1980](47) Not available. Not available.
  Styles et al. [1982](59) Red Signal Ahead signs reduced:

• Right-angle accident rates by 42 percent on approaches with sharp vertical crests.

• Total accident rates on horizontal curve approaches by 14 percent.

• Total accident rates on flat approaches by 41 percent.

Before/After. 20 intersections. High-speed signalized intersection approaches.
Advance warning signs (continued) Styles et al. [1982](60) All intersections showed a reduction in right-angle accidents with the implementation of a flashing red strobe light. One of the intersections showed reductions in right-angle, rear-end, and total accidents of 83 percent, 60 percent, and 61 percent, respectively. Before/After. Four intersections in Maryland. High-speed signalized intersection approaches.
Lighting Bauer and Harwood [1996](20) At rural, four-leg, STOP-controlled intersections, lighted intersections had 21 percent fewer total and injury accidents than unlighted intersections. However, no similar effect was observed for total intersection accidents, and an effect in the opposite direction, indicating that lighted intersections had more accidents than unlighted intersections, was observed for urban four-leg STOP-controlled intersections. These results were based on accidents for all times of day (daytime plus nighttime). Statistical modeling with negative binomial regression. 2,262 rural four-leg STOP-controlled intersections and 1,551 urban four-leg STOP-controlled intersections. At-grade intersections on California state highways.
Box [1989](61) Lighting improvements along a suburban arterial street were found to reduce the percentage of total intersection accidents that occur at night as follows:

Accident Percent reduction

• Pedestrian/bicycle 50 to 33

• Fixed object 57 to 25

• Sideswipe 32 to 11

• Other 33 to 25

Only nighttime head-on accidents increased as a proportion of total head-on accidents from 33 percent to 43 percent. There is no indication whether any of the observed changes in nighttime accident proportions are statistically significant.

Before/after comparison of the proportion of nighttime accidents. Two years of accident data before and Two years after the improvement. One 2.8-mi suburban arterial highway in Naperville (suburban Chicago, Illinois).
Lighting (continued) City of Los Angeles [1980](62) Found no statistically significant reduction in nighttime accidents due to lighting improvements at intersections. Significant reductions in nighttime accidents were found for a few intersections. The assessment of this study is based on review by Keck [1990].(70) Before/after evaluation based on regression analysis. 528 urban intersections; 2 years of data for nighttime accidents and persons injured at the study intersections were used. Urban intersections on city streets in Los Angeles, California.
Traffic Characteristics
Average daily traffic

• Total entering ADT, all approaches

Many studies used total entering ADT as traffic exposure in their studies (e.g., to calculate intersection accident rates).        
  Hauer [1988](15) Explicitly examined safety effects of signalized intersections on traffic flow impacts. He concluded that logically sound models require that the accidents be related to the traffic flows to which the colliding vehicles belong and not the sum of the entering volumes. Comparative and review of past studies. Sample data from past studies and simulated data. Mostly urban settings.
  Lau and May [1988](39) The total entering ADT on all approaches combined was found to have a statistically significant relationship to injury accidents at signalized intersections. Linear regression model. 2,488 signalized intersections. Seven years of injury accident data for each intersection. Signalized intersections on California state highways.
  Lau and May [1988](40) The total entering ADT on all approaches combined was found to have a statistically significant relationship to injury accidents at unsignalized intersections. Linear regression model. 17,000 unsignalized intersections. Seven years of injury accident data for each intersection. Unsignalized intersections on California state highways.
Average daily traffic

• Entering ADTs for major and minor approaches

Bauer and Harwood [1996](20) Major-road and crossroad ADT variables were present in all models as significant predictors of accident frequency. The relative effects for major-road ADT ranged from 1.77 to 2.68, depending on intersection settings (rural/urban, four-leg/three-leg, signalized/STOP-controlled). The relative effects of minor-road ADT ranged from 1.24 to 1.80. Statistical modeling with negative binomial regression. 14,432 intersections in California. Rural and urban, four-leg and three-leg intersections, STOP-controlled and signalized. Intersection major-road ADT above 400 veh/day, minor-road ADT above 100 veh/day.
  Lau and May [1988](39) Percentage of total entering traffic on crossroad was found to be a significant factor in predicting injury accidents at signalized intersections. CART analysis of residuals from base model. 2,400 signalized intersections. Seven years of injury accident data for each intersection. Signalized intersections on California state highways.
  Lau and May [1988](40) Percentage of total entering traffic on crossroad was found to be a significant factor in predicting injury accidents at unsignalized intersections. CART analysis of residuals from base model. 17,000 unsignalized intersections. Seven years of injury accident data for each intersection. Unsignalized intersections on California state highways.
Turning movements Hauer et al. [1988](63) Developed relationships between accident frequency for specific accident types (e.g., left turn) and associated turning volumes. Predictive modeling with negative binomial regression. Three years of accident data for 145 intersections in Toronto, Ontario, Canada; turning movement data by approach and time of day. Urban, four-leg, fixed-time, signalized intersections with two-way traffic on all approaches and no turn restrictions.
Peak hour approach volumes None found        
Vehicle mix / percent trucks None found        
Distribution of total entering volume by hour of the day None found        
Distribution of approach volume by hour of the day None found        
Average approach speed None found        
Volume of bicycle traffic None found        
Volume of pedestrian traffic None found        

 

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