The Highway Safety Information System

(HSIS) is a multi-State safety data base that

contains accident, roadway inventory, and traffic volume data for a select group of States. The

participating States, California, Illinois, Maine,

Michigan, Minnesota, North Carolina, Utah, and

Washington, were selected based on the quality of

their data, the range of data available, and their

ability to merge data from the various files. The HSIS

is used by FHWA staff, contractors, university

researchers, and others to study current highway safety issues, direct research efforts, and evaluate the effectiveness of accident countermeasures. HSIS

HIGHWAY

S AFETY

I NFORMATION

S YSTEM

Research, Development & Technology 

Turner-Fairbank Highway Research Center

6300 Georgetown Pike • McLean, Virginia 22101-2296

SUMMARY REPORT

GIS-Based Crash Referencing

and Analysis System

A geographical information system (GIS) can be simply defined as a collection

of hardware and software that is used to edit, analyze, and display geographical

information stored in a spatial data base. In recent years, many transportation

departments and other related organizations, such as the Federal Highway

Administration (FHWA), have examined the feasibility of using GIS for transportation planning, systems management, and engineering applications. In

some States and municipalities, GIS is being used to plan transportation

routes, manage pavement and bridge maintenance, and perform a variety of

other traditional transportation-related functions. One area where GIS has

yet to be extensively applied is in the analysis of crash data.

Computerized crash analysis systems in which crash data, roadway inventory data, and traffic operations data can be merged are used in many

States and municipalities to identify problem locations and assess the

effectiveness of implemented countermeasures. By integrating this traditional system with a GIS, which offers spatial referencing capabilities

and graphical displays, a more effective crash analysis program can be

realized. The objective of this effort was to develop a crash referencing

and analysis system within a GIS.

DATA BASES USED

The system was developed using the Highway Safety Information

System (HSIS) for North Carolina for the area of Wake County. It

provides the functions needed to edit tabular and spatial crash and

roadway data and to perform crash analysis. The HSIS data bases

incorporated into the system include the crash and roadway

inventory files. Additional data bases maintained by the North

Carolina Department of Transportation (DOT) that were integrated into the system included the traffic signal inventory, pavement management/condition inventory, railroad grade crossing

inventory, and the average daily traffic file. Also included in the

system are data bases that are not traditionally used in safety

analyses, such as the land-use and zoning ordinance files for

Wake County and the TIGER census files containing population and demographic information.

SYSTEM DESIGN

With built-in GIS functionality and the ability to customize

the graphical user interface, ArcView GIS was chosen as

the primary software for the system, with ARC/INFO as a

supplement.(1) The system includes a user interface that

allows data to be entered, edited, analyzed, and exported

to other applications. It also features graphical displays,
point-and-click selections, pull-down and pop-up

menus, the ability to view multiple windows concurrently as shown in figure 1, and a minimum

number of typed command requirements.

The system is designed with three principal functions: file/report management, crash location editing, and data analysis. Management tools allow

for the import and export of data files, viewing

and extraction of analysis reports, and deletion of

files and programs used in previous analysis

efforts. The editing function is provided to handle

crashes that have been mileposted incorrectly or

not mileposted at all. The user can select the crash

to be mileposted and through the graphical interface simply position the crash at the correct location using the mouse.

The system is also supplemented with the capability of viewing scanned crash reports and videologs. The crash report viewer allows the user to

examine images of reports, including narratives

and collision diagrams, that have been scanned

into the system, and includes zoom and pan capabilities. The videolog viewer allows

the user to enter a route and milepost, and to access the video images for that location. These images were obtained for both directions of travel and can be used to

assess geometric features, signs and markings, and other relevant information.

CRASH ANALYSIS TOOLS

The analysis tools include five separate programs to evaluate crashes at designated

spots or intersections, along specific roadway segments or strips, clustered around

a specific roadway feature, or within a defined corridor. All of the programs allow

the user to produce results for all crashes or for a given subset of crashes that can

be defined using any of the variables contained in the crash and roadway inventory files. For example, if the analysis issue of interest involved only nighttime

crashes, the light condition variable could be used to include only those crashes

occurring when it was dark. The user can also perform additional ad hoc

queries using the analysis and mapping tools in ArcView.

The Spot/Intersection Analysis program is used to evaluate crashes at a

user-designated spot or intersection within a given search radius. The end

result of this analysis is a report that lists the number of crashes by injury

severity, crash cost, and other variables designated by the user. A graphic

depicting the spot or intersection, search radius, selected crashes, and roadway identifiers is also produced and can be output as a hardcopy map as

shown in figure 2.

The Strip Analysis program is used to study crashes along a designated

length of roadway as opposed to a spot or intersection. The user must

enter the route(s) of interest and the length of the segment to be used in

the analysis. For example, if the user input 0.50 km as the segment

length, the program would analyze each consecutive 0.50-km segment

along each route, from 0.00 to 0.50, 0.51 to 1.00, etc. The end result of

this analysis is a report with similar output to that produced for the

intersection analysis and a graphic that can also be output as a hardcopy map depicting the strip, selected crashes, and roadway identifiers as shown in figure 3.

Figure 1. The GIS-based crash 

referencing and analysis system.
The Cluster Analysis program is used to study crashes

clustered around a given roadway feature such as a

bridge, railroad crossing, or traffic signal. Crashes within a user-defined distance of a feature are identified on

all routes selected. If the number of crashes found is

equal to or greater than the user-defined minimum

threshold, the location is flagged and included in the

results. The analysis report lists various summary statistics based on user inputs and produces a graphic that

can be printed as a hardcopy map depicting the high

crash locations.

The Sliding-Scale Analysis program is used to identify

roadway segments with a high crash occurrence. This

program differs from the Strip Analysis program in that

the analysis segment is not fixed, but rather slides along

the route in an incremental fashion. The user defines the

segment length and the increment length for analysis.

For example, if the segment length is defined as 0.50 km

and the increment length is 0.10 km, the program starts

at the beginning of a route and analyzes the first segment

from 0.00 to 0.50 km, then slides 0.10 km and analyzes

the second segment from 0.10 to 0.60 km. If the crash

rate of any strip meets or exceeds the user-defined

threshold, the segment is extended by the incremental

distance, and the process is repeated. A segment is ended

when the maximum milepost of the route is reached or

when the user-defined number of allowable extensions

without a crash is exceeded. Based on other user-defined

parameters, intersections can either be ignored, excluded based on a buffer around each intersection, or included without any buffer and inclusive of crashes within a

specified distance on intersecting routes. The end result

of the analysis includes a table showing the high crash

locations, along with a variety of summary statistics and

a map showing these locations.

The Corridor Analysis program is used to locate high

crash concentrations within a corridor. Using traditional methods, segments along a specific route could be

examined (e.g., by using the Sliding-Scale Analysis), but

multiple routes within a corridor could not be easily

linked and analyzed as a group. This program allows

routes to be linked together in a manner that allows the

analyst to assess the overall safety performance within a

transportation corridor. In this current effort, the program was used to examine truck crashes along designated truck corridors

in Wake County. State laws permit trucks to drive on any designated

truck route and along any intersecting routes for a distance of up to 3 mi

(4.8 km). These routes and distances define the truck corridors. Using

the Sliding-Scale Analysis program and the Corridor Analysis program

developed to identify truck crashes within the 3-mi (4.8-km) driveable

zones, high truck crash zones within the corridors were identified. The

output of the analysis included crash statistics and a variety of roadway characteristics for each high crash zone in the corridor. In addition, a plot depicting the roadways included and the crash locations in

each zone was also produced. A future HSIS summary report will

provide a complete overview of the truck corridor study.

Roads

Search Radius 1 2 Mi e

M epos d C ashes

Se ed pot Cra hes

/ l

i te r

lect S s

0 2 0 .2 Miles . 0 l

40002006

20000401

Figure 2. Results from the Spot/

Intersection Analysis program can be

graphically displayed as shown here.

10000440

40001321

10000040

40001319

40001009

40001418

40001349

Raleigh

Mileposted Crashes

Selected Strip Crashes

Buffer Distance 1/10 of a Mile

Roads

0.2 0 0.2 Miles 

Figure 3. Results from the Strip

Analysis program can be graphically displayed as shown here.
This GIS-based crash referencing and analysis system enhances the capability of

engineers to conduct problem identification and countermeasure evaluation studies. Specifically, the advantages of this system over the more traditional referencing and analysis methods include the ability to:

◆ Quickly milepost a crash that was mileposted incorrectly or had no milepost

provided.

◆ Evaluate the problem using the spatial relationships available from the

graphical displays compared to the traditional methods of tables and plots.

◆ Produce presentation graphics and descriptive plots from within the software as opposed to exporting results to another software package for such

production.

◆ Develop an on-line capability to respond to concerns and questions about

specific locations.

◆ More accurately report crashes and roadway features when combined

with global positioning system (GPS) technology, thus making the analysis

more reliable.

◆ Conduct corridor analyses by automatically linking adjacent or nearby

routes together within defined zones.

◆ Incorporate non-traditional data bases that are available within GIS,

such as land use, zoning ordinances, and population characteristics, into

problem identification and evaluation studies.

In addition, the system allows traffic engineers to access various types

of supplemental information without leaving their desks. Scanned versions of an officer’s handwritten crash report can be examined to provide detailed information that may not be contained in the crash

event table. Routes and crashes can be overlaid with scanned aerial

photography to provide the engineer with a better understanding of

the development and roadway configuration of a particular study

area, and images from the videolog can be scrutinized to provide an

even more detailed view of the roadway. Various attribute tables can

be created, edited, and linked to other tables, providing even more

flexibility and power. All of these tools combine to form a system

that makes locating, editing, and analyzing crashes and other spatial data faster and more efficient.

FUTURE DEVELOPMENTS

Recently, FHWA initiated a study to expand on the current

effort conducted in North Carolina to further develop, test, and

integrate the GIS-based crash referencing and analysis system

in other HSIS States. The GIS capabilities of these States are

being examined and the feasibility of integrating the developed system into the State data bases is being explored. Work

is also underway to develop GIS analysis tools to improve

pedestrian and bicycle safety. The pedestrian application

involves the development of a system to identify safe routes

to school, and the bicycle application is exploring the use of

GIS to assess the bicycle compatibility of roadways.

REFERENCE

1.ArcView GIS and ARC/INFO are

GIS software packages developed

by Environmental Systems

Research Institute, Inc., 

Redlands, California.

FOR MORE INFORMATION

This system was funded by FHWA and

was developed by the North Carolina

Center for Geographic Information and

Analysis, with technical assistance from the

North Carolina DOT and the University 

of North Carolina Highway Safety Research

Center (UNC-HSRC). For more information

about the system, contact David Harkey,

UNC-HSRC, at (919) 962-8705. 

For more information about HSIS, contact

Michael Griffith, HSIS Program Manager, 

at (703) 285-2382, mike.griffith@fhwa.dot.gov.

Publication No. FHWA-RD-99-081

February 1999

visit our web site at

www.tfhrc.gov