Procedure for curve warning signing, delineation, and advisory speeds for horizontal curves : final report - Page 80 |
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69
A subsequent but separate sensitivity analysis of the radii‘s accuracy in relation to the number of GIS points that were used to define the curve itself found that the accuracy of Curve Calculator increased as the number of GIS points increased. The Curve Calculator error decreased from over 3% with 5 GIS points to approximately only 1% with 8 GIS points to less than 0.1% with 25 GIS points. The Curve Finder error was minor with an error of less than 0.03% on average, but not with a consistent trend in relation to the number of points. There was no error associated with Curvature Extension as it exactly matched the true radius regardless of the number of GIS points.
The field measured curve analysis compared each of the three GIS methods to 51 field measured curves in North Carolina. Curve Finder identified 45 of the 51 curves, while the other two methods identified every curve. Curvature Extension appears to be the most accurate evaluator of curve radius by measuring the radius within 50% of the field measured value on 80% of the curves. Curve Calculator was within 50% of the field measured value for 78% of the curves and Curve Finder measured 69% of the curves to within 50% of the field measurement. Curve Calculator and Curvature Extension were equally proficient in assessing length measurements with 76% of their measurements within 50% of the field measured values. Curve Finder reported only 40% of the curves to within 50% of the field measured length. A 1524 meters (5,000 feet) maximum tolerance was specified in Curve Finder which provided useful results for the curves examined in this study.
Curve safety rankings between the methods were tested using Spearman correlation coefficients. The Field Measured rankings share positive, statistically significant correlation with each method (from 0.88 to 0.90). When comparing the methods to each other, positive, statistically significant correlation is also observed, with the highest correlation between Curve Calculator and Curvature Extension of 0.99 representing a strong relationship between the rankings from the two programs.
A test of the linear correlation of both the radius and length between the methods was conducted to examine the similarity between the results for each method. Overall, many of the methods share positive, statistically significant correlation with each other which supports the reliability of the GIS methods while emphasizing the importance of quality line work for accurate results. The GIS line work showed considerable quality variation in this study ranging from some curves with minute errors to some errors larger than the radius itself. Still, Table 4 shows that even with the line work evaluated in this study, a considerable percentage of radius values were captured depending on the desired level of accuracy.
7.6 Conclusions
The need for horizontal curve information extends to numerous transportation purposes which range from individual curve safety investigations to long-range network planning and beyond to applications in other domains. This analysis found that each of the three GIS programs studied is well suited for specific applications, as discussed below, depending on the user‘s needs and the quality of the GIS line work. The level of accuracy that each of these programs can provide is high enough for most typical safety and planning applications. Many transportation agencies maintain GIS programs and
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| Title | Procedure for curve warning signing, delineation, and advisory speeds for horizontal curves : final report - Page 80 |
| Full Text | 69 A subsequent but separate sensitivity analysis of the radii‘s accuracy in relation to the number of GIS points that were used to define the curve itself found that the accuracy of Curve Calculator increased as the number of GIS points increased. The Curve Calculator error decreased from over 3% with 5 GIS points to approximately only 1% with 8 GIS points to less than 0.1% with 25 GIS points. The Curve Finder error was minor with an error of less than 0.03% on average, but not with a consistent trend in relation to the number of points. There was no error associated with Curvature Extension as it exactly matched the true radius regardless of the number of GIS points. The field measured curve analysis compared each of the three GIS methods to 51 field measured curves in North Carolina. Curve Finder identified 45 of the 51 curves, while the other two methods identified every curve. Curvature Extension appears to be the most accurate evaluator of curve radius by measuring the radius within 50% of the field measured value on 80% of the curves. Curve Calculator was within 50% of the field measured value for 78% of the curves and Curve Finder measured 69% of the curves to within 50% of the field measurement. Curve Calculator and Curvature Extension were equally proficient in assessing length measurements with 76% of their measurements within 50% of the field measured values. Curve Finder reported only 40% of the curves to within 50% of the field measured length. A 1524 meters (5,000 feet) maximum tolerance was specified in Curve Finder which provided useful results for the curves examined in this study. Curve safety rankings between the methods were tested using Spearman correlation coefficients. The Field Measured rankings share positive, statistically significant correlation with each method (from 0.88 to 0.90). When comparing the methods to each other, positive, statistically significant correlation is also observed, with the highest correlation between Curve Calculator and Curvature Extension of 0.99 representing a strong relationship between the rankings from the two programs. A test of the linear correlation of both the radius and length between the methods was conducted to examine the similarity between the results for each method. Overall, many of the methods share positive, statistically significant correlation with each other which supports the reliability of the GIS methods while emphasizing the importance of quality line work for accurate results. The GIS line work showed considerable quality variation in this study ranging from some curves with minute errors to some errors larger than the radius itself. Still, Table 4 shows that even with the line work evaluated in this study, a considerable percentage of radius values were captured depending on the desired level of accuracy. 7.6 Conclusions The need for horizontal curve information extends to numerous transportation purposes which range from individual curve safety investigations to long-range network planning and beyond to applications in other domains. This analysis found that each of the three GIS programs studied is well suited for specific applications, as discussed below, depending on the user‘s needs and the quality of the GIS line work. The level of accuracy that each of these programs can provide is high enough for most typical safety and planning applications. Many transportation agencies maintain GIS programs and |
