Systematic Identification of High Crash Locations

Accident Identifications on horizontal curves

The geographically referenced Iowa roadway base record data and crash records [i.e., the Geographic Information System Accident Analysis and Location System (GISALAS)] is the primary source of information for this project. It contains the location and characteristics of all Iowa crashes during the past ten years. Although, it lacks the information on whether or not a roadway string is a curve in its entirety or contains one or more curves, it contains valuable information leading to identification of horizontal curves. The curve-related information includes:

On-curve accidents (crash records)
Number of curves that have posted advisory speed limit signs (base records)
Base record curve data (This database, which covers about 20 percent of the Iowa roadways, is no longer being updated.)
Bearing of the segments
String length (actual versus end to end straight lengths)

Although the accuracy of the reported on-curve accidents is yet to be determined, this important piece of information has led to development of two strategies to identify horizontal curves in the base records. Other developed strategies are based on the segment bearings and string lengths.

These strategies have been assessed through visual inspections using cartography maps and aerial photos. The quantitative and qualitative assessments of the strategies, presented in Table 1, will assist us in selecting one or a combination of techniques to estimate locations and lengths of the horizontal curves on the Iowa roadway network. Incorporating other curve-related information could also enhance the selected technique(s).

Table 1. Examined Strategies to Identify Horizontal Curves

	Assessment Measures
Strategies	Definition	Quantitative Assessment (%)	Type I Error (%)	Type II Error (%)	Cost	Qualitative Assessment	Procedure	Illustration
Crash (string)	Def	66	23	11	$	QA	Proc	Pic
Crash (segment)	Def	68	30	2	$	QA	Proc	Pic
Bearing-1	Def	Quantitative Assessment			$	QA	Proc	Pic
Manual (string)	Def	85	13	2	$	QA	Proc	Pic
Manual (segment)	Def	72	28	0	$	QA	Proc	Pic
Vertex	Def	51	0	49	$	QA	Proc	Pic
Length (string)	Def	80	19	1	$	QA	Proc	Pic
Length (defined interval)	Def	In-Progress…						Pic

Quantitative Assessment - Accuracy level of the method – one minus type I error plus type II error

Type I Error - H₁ is selected as being correct when H₀ is correct

Type II Error - H₀ is selected as being correct when H₁ is correct

H₀: not a curve
H₁: a curve

Through the visual inspection of the sites’ maps in the GIS environment, each record is marked as being a curve or a line. The discrepancies between the two procedures define the two error types.

Type I error is the percent of records that have been tagged as curves, but the visual inspections indicate differently.

Type II error (i.e., a more serious error) is the percent of records that have been tagged as not curves, but the visual inspections indicate differently.

Fixed Cost - Training and development cost

Marginal Cost - Changes in cost for an additional site study

Qualitative Assessment - What are the implications if we make mistakes