INTERSECTION ACCIDENT ANALYSIS

Accident and safety analysis can be quite thorough and intensive, which is outside the scope of this report. The accident analysis given here is quite preliminary and macroscopic in scale. The purpose of this section is to give the City a guide as to which intersections may warrant further analysis and study in the future. Such studies could evaluate specific intersections in much more detail as well as using other criteria such as types of accidents or time periods. For each type of accident there are several probable causes that may have contributed to the accident. For each probable cause there are several countermeasures that are intended to reduce the probability of those types of accidents occurring in the future. Each countermeasure can be evaluated in terms of the expected overall or percent reduction in accidents at that location.

Accident data was obtained from the West Jordan City Police Department from the beginning of 1999 through the end of 2001. This data provided a summary of the accidents at each of the major intersections in the city over the full three years. A list of all the accidents involving bicyclists or pedestrians was also produced by the Police Department. There are typically five different ways that accidents at intersections can be summarized:

• Type of accident

• Severity

• Contributing circumstances

• Environmental conditions

• Time periods

Due to the nature of the information supplied by the Police Department and the purposes of this report it was determined that the intersections would be evaluated by summarizing the accidents in terms of accident severity. This is a common way of summarizing accidents rates at intersections as well as along highways and streets.

By knowing the number of accidents per year as well as the yearly traffic volume traveling through an intersection an accident rate can be calculated for each intersection. Accident rates represent the total number of accidents occurring for every million vehicles passing through the intersection. Accident rates allow the comparison of many intersections in order to identify those intersections that are more dangerous, or that tend to have more accidents than others relative to the total volume.

00

tt

N e w B i n g h a m H i g h w a y

O l d B i n g h a m H i g h w a y

9 0 0 0 S o u t h 7 8 0 0 S o u t h

6 2 0 0 S o u t h

3200 West 2700 West 1300 West

4000 West Bangerter Highway Redwood Road

SR 111

4800 West

5600 West Airport Road 2200 West

Grizzly Way

7 0 0 0 S o u t h

N O R T H

W e s t J o r d a n M a s t e r T r a n s p o r t a t i o n P l a n

E N G I N E E R S P L A N N E R S E C O N O M I S T S

W i l b u r S m i t h A s s o c i a t e s

R I C H A R D D C H O N G_{A N D}

A S S O C I A T E S

I n t e r P l a n C o .T r a n s p o r t a t i o n P l a n n i n g

E x i s t i n g T r a f f i c S i g n a l s F i g u r e 3 . 5 . 1

EXISTING TRANSPORTATION SYSTEM

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West Jordan Master Transportation Plan

ENGI NEERS PL ANNERS ECON OMI STS Wilbur Smith Associates

RICHARD D CHONG AND AS SOCIATE S InterPlan Co.

Tran spo rta tio n Pla nn in g

These rates can also be broken down in terms of fatalities, injuries, and property damage only-types of accidents. Typically a weighting factor is used for each of these only-types of rates based on their severity. There are no absolute weighting criteria but a typical scale gives a weighting factor of 1 for property damage only, 3 for injuries, and 8 for fatalities. Each of the categorical accident rates are multiplied by the weighting factor and summed up to determine a composite score for each location. The higher the score, the more dangerous that intersection is. The ten intersections with the highest composite score are shown in Table 3.6.1. The complete data and calculations are found in the Appendix.

Table 3.6.1 – Accident Data for the Ten Most Dangerous Intersections

Weighting Factor 1 3 8

Intersection

Property-Only Injury Fatal Total Rate Composite Score Old Bingham & 4000 W. 1.99 1.61 0.00 3.66 6.82 7800 S. & Airport Rd. 1.11 1.05 0.00 2.16 4.26 7000 S. & 1300 W. 0.69 0.86 0.06 1.64 3.72 New Bingham & 4800 W. 1.23 0.76 0.00 1.99 3.50 Old Bingham & 4800 W. 1.16 0.56 0.00 1.72 2.83 7800 S. & 1300 W. 0.83 0.52 0.00 1.37 2.39 9000 S. & Bangerter Hwy. 0.61 0.56 0.00 1.22 2.29 7800 S. & 2700 W. 1.00 0.43 0.00 1.46 2.26 9000 S. & 4000 W. 0.72 0.47 0.00 1.22 2.13 7000 S. & 3200 W. 0.38 0.56 0.00 0.94 2.05

Note: Accident Rates are given in terms of number of accidents per 1 million vehicles entering through the intersection.

Once these ten intersections were identified they were evaluated in more detail to determine types of accidents and possible causes. Some of these intersections have had changes in traffic control devices either during this accident study period or since then. In some cases these recent improvements may reduce the number of accidents at that location which could be evaluated after a sufficient amount of time has passed. The results of any safety improvements or countermeasures must be evaluated over time to know of their effects. Each of these intersections and some accident reduction countermeasures are given below. Some types of studies that would be useful for a more detailed analysis of each location are also given in the table.

EXISTING TRANSPORTATION SYSTEM

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West Jordan Master Transportation Plan

ENGI NEERS

Table 3.6.2 – Possible Causes and Mitigation Measures

Intersection Probable Cause General Countermeasure Additional Procedures that May Be Performed

Large volume of left turns, Large total Travel Time and Delay study

Spot Speed study

1Traffic signal, Turn lanes

Volume study Traffic Conflict study Roadway Inventory study

Capacity study Travel Time and Delay study

Spot Speed study

1Traffic signal, Turn lanes

Volume study Traffic Conflict study Roadway Inventory study

Capacity study Travel Time and Delay study

Spot Speed study Gap study 7800 South

& 1300 West

Large volume of left turns, Large total intersection volume

2Widen 7800 South

Volume study Traffic Conflict study Roadway Inventory study

Capacity study Travel Time and Delay study 9000 South of red light running and very aggressive driving Travel Time and Delay study

EXISTING TRANSPORTATION SYSTEM

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West Jordan Master Transportation Plan

ENGI NEERS

Intersection Probable Cause General Countermeasure Additional Procedures that May Be Performed

9000 South

& 4000 West

Large volume of left turns, Large total arms with 12-in. lenses

Volume study Roadway Inventory study

Traffic Conflict study Capacity study Travel Time and Delay study

Traffic Control Device study

7000 South

& 3200 West

Large volume of left turns, Large total Travel Time and Delay study

1Indicates improvement measure has been implemented recently.

2Indicates improvement will be constructed in near future.

A compilation entitled Intersection Safety Briefing Sheets produced jointly by the Federal Highway Administration (FHWA), the Advocates for Highway and Auto Safety, and the Institute of Transportation Engineers (ITE) gives some excellent guidelines on various intersection safety related topics. This document discusses various topics related to safety problems, their causes, and effects of various mitigation efforts. This is an excellent resource in understanding intersection safety for all individuals concerned with traffic safety, not just traffic engineering professionals.