Safety Evaluation of the Safety Edge Treatment
Chapter 3. Preliminary Analysis Results for Field
Measurements of PavementEdge DropOffs
This chapter presents preliminary analysis results for field
measurements of pavement‑edge drop‑offs. Field measurements of
dropoff heights were made to evaluate the comparability of existing pavementedge
dropoffs for the treatment and comparison sites in the period before
resurfacing and to verify that the safety edge treatment does not encourage the
development of pavementedge dropoffs in the period after resurfacing.
Field data for pavementedge dropoff heights were collected
for each participating agency for both treatment and comparison sites in the
period before resurfacing and during each year after resurfacing. The field
data collection methodology is presented in appendix B. A few sites were
resurfaced before field visits could be made. Consequently, these sites were
excluded from the analysis of beforeperiod dropoff height data presented in
this chapter.
3.1 Comparison of DropOff Measurements for Treatment and Comparison Sites Before
Resurfacing
A formal assessment of the comparability of the treatment and
comparison sites with respect to the presence of pavementedge dropoffs in the
period before resurfacing was undertaken. The measure used for this comparison
was the proportion of dropoff heights exceeding 2 inches. This criterion was used based on research
indicating that pavementedge dropoff heights exceeding 2 inches may affect
safety.^{(1)} It should be noted that this previous research was
conducted on sites without the safety edge treatment.
It would be desirable if the proportion of sites with pavementedge
dropoff heights exceeding 2 inches were similar for the treatment and
comparison sites in the period before resurfacing. An analysis to make this comparison was conducted by performing a logistic
regression analysis using the LOGISTIC procedure in SAS^{®}.^{(3)} This procedure uses the Fisher scoring method to estimate the statistical significance of differences in
proportions between the treatment and comparison sites.
Ideal results for this analysis would have been obtained if
the difference between the proportions of
dropoff heights exceeding 2 inches for the treatment and comparison sites were
not statistically significant at some
predetermined significance level. A statistically significant result would be
indicated by an odds ratio point estimate
that was significantly greater than or less than 1.0 (i.e., the confidence interval for the odds ratio does not
contain 1.0). Conversely, for a difference that is not statistically
significant, the odds ratio for the difference would contain 1.0. If the odds
ratio could not be determined by maximum
likelihood due to small sample size or poor variation of responses (i.e.,
identical responses for each site type or nonoverlapping responses between
site types), then an exact test was performed and a median unbiased estimate of
the odds ratio was provided.
The results of this
analysis for each State, roadway type, shoulder type, and treatment type combination,
including the frequency and proportion of measurements exceeding 2 inches, the odds ratio point estimate, the odds ratio
confidence interval, and the statistical significance of the odds ratio point estimate, are given in table
6. Odds ratio values above 1.0 in this table indicate that
comparison sites had a greater probability of experiencing dropoffs exceeding
2 inches than treatment sites.
Table 6. Comparison of the proportions of dropoff heights exceeding 2 inches for the period before resurfacing.
State 
Roadway type 
Shoulder type 
Site type 
Dropoff heights that exceed 2 inches 
Odds ratio point estimate 
Lower confidence limit 
Upper confidence limit 
Statistically significant at 0.05 level? 
Number 
Proportion 
GA 
Multilane 
Paved 
T 
2 
0.07 
0.909 
0.184 
6.596 
No 
C 
5 
0.06 
Twolane 
Paved 
T 
10 
0.03 
4.591 
2.211 
10.259 
Yes 
C 
25 
0.14 
Unpaved 
T 
23 
0.09 
1.557 
0.876 
2.799 
No 
C 
29 
0.13 
IN 
Twolane 
Paved 
T 
6 
0.04 
2.519 
0.902 
7.642 
No 
C 
10 
0.10 
Unpaved 
T 
150 
0.39 
0.423 
0.291 
0.608 
Yes 
C 
53 
0.22 
NY 
Twolane 
Paved 
T 
36 
0.38 
0.028 
0.000 
1.620 
No^{1} 
C 
0 
0.00 
^{1} Indicates that median unbiased estimate
was used.
T = Treatment sites
resurfaced with safety edge.
C = Comparison sites resurfaced without
safety edge.
The results in table
6 indicate that in the period before resurfacing, there
were relatively equal proportions of extreme
dropoff heights between treatment and comparison sites for Georgia sites
on multilane highways with paved shoulders
and twolane highways with unpaved shoulders. This finding indicates that these
two types of sites were relatively well matched in terms of shoulder conditions in the period before
resurfacing. By contrast, the findings for Georgia sites
on twolane highways with paved
shoulders suggest that there was a statistically significant chance that
comparison sites had greater proportions of dropoffs exceeding 2 inches.
For Indiana sites on twolane highways with paved shoulders,
there was a greater proportion of extreme dropoff heights for the comparison
sites than for the treatment sites in the period before resurfacing, but the difference was not statistically significant. The
opposite was the case for Indiana sites
on twolane highways with unpaved shoulders and for New York sites on twolane
highways with paved shoulders. In
these cases, the treatment and comparison sites were not perfectly matched in
terms of shoulder conditions in the period before resurfacing. For Indiana,
this difference was statistically
significant. Some differences of this sort may have been inevitable because
resurfacing projects that received the safety edge treatment were not
selected based on consideration of the existing shoulder condition. This is a
potential confounding factor that should be considered in interpreting the
research results.
3.2 Comparison of DropOff Measurements between the Before and After Resurfacing periods
The field measurement data for pavementedge
dropoffs were initially reviewed by State, roadway type, shoulder type,
and treatment type. Table
7 presents summary descriptive statistics for these measures for each study period. Figure 2 presents histograms for a sample of the distributions
and shows the impact of resurfacing for both treatment and comparison
sites.
Table 7. Summary of pavementedge dropoff height measurements.
State 
Road type 
Shoulder type 
Site type 
Before resurfacing 
After resurfacing (Year 1) 
Number of measurements 
Dropoff height (inches) 
Coefficient of variation % 
Number of measurements 
Dropoff height (inches) 
Coefficient of variation % 
Minimum 
Mean 
Median 
Maximum 
Standard deviation 
Minimum 
Mean 
Median 
Maximum 
Standard deviation 
GA 
Multilane 
Paved 
T 
30 
0 
0.783 
0.750 
2.000 
0.618 
79 
59 
0.375 
1.047 
0.875 
2.875 
0.504 
48 
C 
82 
0 
0.811 
0.750 
3.000 
0.710 
88 
86 
0.250 
1.038 
1.000 
2.375 
0.467 
45 
Twolane 
Paved 
T 
291 
0 
0.546 
0.500 
3.750 
0.611 
112 
289 
0.000 
0.960 
1.000 
2.375 
0.495 
52 
C 
178 
0 
0.912 
0.750 
4.000 
0.912 
100 
150 
0.000 
0.887 
0.875 
1.875 
0.471 
53 
Unpaved 
T 
270 
0 
0.881 
0.750 
3.750 
0.695 
79 
273 
0.000 
0.941 
0.875 
2.500 
0.495 
53 
C 
229 
0 
1.076 
1.000 
4.750 
0.804 
75 
466 
0.000 
0.945 
0.875 
2.875 
0.556 
59 
IN 
Twolane 
Paved 
T 
136 
0 
0.630 
0.500 
3.500 
0.598 
95 
158 
0.000 
0.703 
0.625 
1.875 
0.356 
51 
C 
96 
0 
0.960 
0.750 
3.250 
0.708 
74 
137 
0.250 
1.340 
1.125 
4.250 
0.707 
53 
Unpaved 
T 
380 
0 
1.758 
1.625 
5.125 
0.778 
44 
367 
0.250 
1.653 
1.500 
4.500 
0.737 
45 
C 
245 
0 
1.353 
1.250 
6.875 
0.930 
69 
279 
0.125 
1.168 
1.000 
5.250 
0.673 
58 
NY 
Twolane 
Paved 
T 
94 
0 
1.681 
1.500 
5.125 
1.270 
76 
77 
0.000 
1.110 
0.875 
4.000 
0.886 
80 
C 
42 
0 
0.777 
0.750 
1.750 
0.487 
63 
83 
0.000 
1.065 
1.000 
2.750 
0.480 
45 
State 
Road type 
Shoulder type 
Site type 
After resurfacing (Year 2) 
After resurfacing (Year 3) 
Number of measurements 
Dropoff height (inches) 
Coefficient of variation % 
Number of measurements 
Dropoff height (inches) 
Coefficient of variation % 
Minimum 
Mean 
Median 
Maximum 
Standard deviation 
Minimum 
Mean 
Median 
Maximum 
Standard deviation 
GA 
Multilane 
Paved 
T 
65 
0.500 
1.175 
1.000 
3.000 
0.448 
38 
65 
0.500 
1.175 
1.000 
3.000 
0.448 
38 
C 
86 
0.250 
0.906 
0.813 
2.500 
0.455 
50 
86 
0.250 
0.907 
0.875 
2.500 
0.442 
49 
Twolane 
Paved 
T 
212 
0.000 
0.956 
0.875 
2.250 
0.455 
48 
254 
0.000 
1.087 
1.000 
3.375 
0.432 
40 
C 
152 
0.375 
1.166 
1.125 
2.250 
0.356 
31 
164 
0.250 
1.104 
1.125 
2.250 
0.372 
34 
Unpaved 
T 
238 
0.125 
1.179 
1.000 
3.563 
0.571 
48 
259 
0.250 
1.107 
1.000 
3.563 
0.566 
51 
C 
426 
0.000 
1.163 
1.125 
3.250 
0.548 
47 
448 
0.000 
1.119 
1.063 
3.250 
0.526 
47 
IN 
Twolane 
Paved 
T 
187 
0.000 
0.788 
0.750 
2.250 
0.379 
48 
189 
0.125 
0.780 
0.750 
2.250 
0.398 
51 
C 
102 
0.250 
1.456 
1.250 
4.375 
0.857 
59 
147 
0.000 
1.344 
1.250 
3.875 
0.609 
45 
Unpaved 
T 
370 
0.250 
1.916 
1.750 
6.875 
0.993 
52 
373 
0.250 
1.584 
1.375 
4.500 
0.774 
49 
C 
280 
0.000 
1.353 
1.250 
5.500 
0.764 
56 
290 
0.125 
1.236 
1.125 
4.500 
0.676 
55 
NY 
Twolane 
Paved 
T 
78 
0.375 
1.786 
1.344 
5.125 
1.191 
67 
78 
0.375 
1.786 
1.344 
5.125 
1.191 
67 
C 
81 
0.625 
1.446 
1.375 
3.250 
0.497 
34 
81 
0.625 
1.446 
1.375 
3.250 
0.497 
34 
T = Treatment sites resurfaced with safety
edge.
C = Comparison sites resurfaced without
safety edge.
Figure 2. Graph. Dropoff height measurement distributions for twolane highways with paved shoulders in Georgia.
In each graph shown in figure
2, the vertical blue line marks a 2inch dropoff
height. The mean dropoff height did not vary between the before and after
periods. For almost all roadway type/ shoulder type/treatment type
combinations, the coefficient of variation (i.e., relative standard deviation) of dropoff height decreased
substantially between before resurfacing and each of the first 2 years after resurfacing
but increased again following the second year after resurfacing.
To formally assess whether
the safety edge treatment has any effect on pavementedge dropoffs, a trend analysis evaluating the change in
dropoffs from before to after resurfacing was conducted. Specifically, the proportion of dropoff height
measurements exceeding 2 inches was evaluated to determine if there were
differences between the before and after study periods. This analysis was
carried out using the same logistic regression approach presented in section 3.1.
However, in this case, the proportions of
dropoff heights exceeding 2 inches were compared between the periods before and after resurfacing for each type of site
rather than between treatment and comparison sites.
The ideal trend for this analysis would be indicated by a substantial
decrease in dropoff height for the first year after resurfacing, possibly
followed by a slow increasing trend in the later years back to the dropoff
height that existed before resurfacing. To evaluate this trend, all pairwise
comparisons between years were evaluated for statistical significance. Four of
the comparisons: before versus after year 1, after year 1 versus after year 2, after
year 2 versus after year 3, and before versus after year 3 have been
summarized.
For before versus after year 1, an odds ratio point estimate less
than 1.0 indicates that after year 1 had more dropoff heights exceeding 2 inches
than the period before resurfacing. A confidence interval for the odds ratio
that does not contain the value 1.0 indicates statistical significance. Since
the odds ratios were less than 1.0 in 3 of the 12 cases shown in table
8, the sites in after year 1 generally had fewer
dropoff heights exceeding 2 inches than the sites in the period before resurfacing. Also, the three cases when after
year 1 had more dropoff heights exceeding 2 inches than the period before resurfacing were not
significant. Thus, it appears that resurfacing tends to reduce the
proportion of extreme dropoff heights.
Table 8. Comparison of the proportions of dropoff heights
exceeding 2 inches between
the before and after resurfacing periods.
State 
Roadway type 
Shoulder type 
Site Type 
Test 
Proportion Period 1 
Proportion Period 2 
Odds ratio point estimate 
Lower 95% confidence limit 
Upper 95% confidence limit 
Statistically significant at the
0.05 level? 
GA 
Multilane 
Paved 
C 
Period Before vs
AfterY1 
0.06 
0.06 
1.05 
0.28 
3.92 
No 
C 
Period Before vs
AfterY2 
0.06 
0.03 
1.80 
0.43 
8.99 
No 
C 
Period Before vs
AfterY3 
0.06 
0.08 
0.73 
0.21 
2.39 
No 
C 
Period AfterY1 vs
AfterY2 
0.06 
0.03 
0.59 
0.12 
2.46 
No 
C 
Period AfterY1 vs
AfterY3 
0.06 
0.08 
1.44 
0.44 
5.03 
No 
C 
Period AfterY2 vs
AfterY3 
0.03 
0.08 
2.45 
0.66 
11.68 
No 
T 
Period Before vs
AfterY1 
0.07 
0.07 
0.98 
0.13 
5.35 
No 
T 
Period Before vs
AfterY2 
0.07 
0.08 
0.86 
0.12 
4.25 
No 
T 
Period Before vs
AfterY3 
0.07 
0.09 
0.70 
0.10 
3.27 
No 
T 
Period AfterY1 vs
AfterY2 
0.07 
0.08 
1.15 
0.29 
4.83 
No 
T 
Period AfterY1 vs
AfterY3 
0.07 
0.09 
1.40 
0.38 
5.72 
No 
T 
Period AfterY2 vs
AfterY3 
0.08 
0.09 
1.22 
0.35 
4.44 
No 
Twolane 
Paved 
C 
Period Before vs
AfterY1 
0.14 
0 
infinity 
12.13 
infinity 
Yes 
C 
Period Before vs
AfterY2 
0.14 
0.05 
3.38 
1.49 
8.70 
Yes 
C 
Period Before vs
AfterY3 
0.14 
0.03 
6.17 
2.33 
21.32 
Yes 
C 
Period AfterY1 vs
AfterY2 
0 
0.05 
infinity 
3.24 
infinity 
Yes 
C 
Period AfterY1 vs
AfterY3 
0 
0.03 
infinity 
1.60 
infinity 
Yes 
C 
Period AfterY2 vs
AfterY3 
0.05 
0.03 
0.55 
0.14 
1.86 
No 
T 
Period Before vs
AfterY1 
0.03 
0.03 
1.11 
0.44 
2.83 
No 
T 
Period Before vs
AfterY2 
0.03 
0.02 
1.85 
0.61 
6.82 
No 
T 
Period Before vs
AfterY3 
0.03 
0 
10.64 
2.02 
195.83 
Yes 
T 
Period AfterY1 vs
AfterY2 
0.03 
0.02 
0.60 
0.16 
1.86 
No 
T 
Period AfterY1 vs
AfterY3 
0.03 
0 
0.10 
0.01 
0.56 
Yes 
T 
Period AfterY2 vs
AfterY3 
0.02 
0 
0.17 
0.01 
1.19 
No 
Unpaved 
C 
Period Before vs
AfterY1 
0.13 
0.06 
2.36 
1.36 
4.10 
Yes 
C 
Period Before vs
AfterY2 
0.13 
0.1 
1.29 
0.78 
2.12 
No 
C 
Period Before vs
AfterY3 
0.13 
0.08 
1.68 
0.99 
2.84 
No 
C 
Period AfterY1 vs
AfterY2 
0.06 
0.1 
1.83 
1.11 
3.04 
Yes 
C 
Period AfterY1 vs
AfterY3 
0.06 
0.08 
1.40 
0.83 
2.38 
No 
C 
Period AfterY2 vs
AfterY3 
0.1 
0.08 
0.77 
0.48 
1.23 
No 
T 
Period Before vs
AfterY1 
0.09 
0.03 
2.73 
1.28 
6.34 
Yes 
T 
Period Before vs
AfterY2 
0.09 
0.13 
0.62 
0.35 
1.10 
No 
T 
Period Before vs
AfterY3 
0.09 
0.09 
0.99 
0.53 
1.85 
No 
T 
Period AfterY1 vs
AfterY2 
0.03 
0.13 
4.39 
2.13 
9.99 
Yes 
T 
Period AfterY1 vs
AfterY3 
0.03 
0.09 
2.76 
1.28 
6.46 
Yes 
T 
Period AfterY2 vs
AfterY3 
0.13 
0.09 
0.63 
0.35 
1.12 
No 
See notes at end of table.
Table 8. Comparison of the proportions of dropoff heights exceeding 2 inches between the before and after resurfacing periodsContinued.
State 
Roadway type 
Shoulder type 
Site Type 
Test 
Proportion Period 1 
Proportion Period 2 
Odds ratio point estimate 
Lower 95% confidence limit 
Upper 95% confidence limit 
Statistically significant at the
0.05 level? 
IN 
Twolane 
Paved 
C 
Period Before vs
AfterY1 
0.10 
0.17 
0.58 
0.25 
1.24 
No 
C 
Period Before vs
AfterY2 
0.10 
0.27 
0.31 
0.13 
0.66 
Yes 
C 
Period Before vs
AfterY3 
0.10 
0.1 
0.70 
0.30 
1.52 
No 
C 
Period AfterY1 vs
AfterY2 
0.17 
0.27 
1.88 
1.01 
3.53 
Yes 
C 
Period AfterY1 vs
AfterY3 
0.17 
0.14 
0.83 
0.43 
1.57 
No 
C 
Period AfterY2 vs
AfterY3 
0.27 
0.14 
0.44 
0.23 
0.83 
Yes 
T 
Period Before vs
AfterY1 
0.04 
0 
infinity 
3.18 
infinity 
Yes 
T 
Period Before vs
AfterY2 
0.04 
0.01 
8.58 
1.44 
163.10 
Yes 
T 
Period Before vs
AfterY3 
0.04 
0.02 
2.86 
0.74 
13.75 
No 
T 
Period AfterY1 vs
AfterY2 
0.00 
0.01 
infinity 
0.15 
infinity 
No 
T 
Period AfterY1 vs
AfterY3 
0.00 
0.02 
infinity 
0.94 
infinity 
No 
T 
Period AfterY2 vs
AfterY3 
0.01 
0.02 
3.00 
0.38 
60.92 
No 
Unpaved 
C 
Period Before vs
AfterY1 
0.22 
0.11 
2.21 
1.37 
3.61 
Yes 
C 
Period Before vs
AfterY2 
0.22 
0.16 
1.48 
0.95 
2.31 
No 
C 
Period Before vs
AfterY3 
0.22 
0.14 
1.68 
1.07 
2.64 
Yes 
C 
Period AfterY1 vs
AfterY2 
0.11 
0.16 
1.49 
0.91 
2.46 
No 
C 
Period AfterY1 vs
AfterY3 
0.11 
0.14 
1.32 
0.80 
2.18 
No 
C 
Period AfterY2 vs
AfterY3 
0.16 
0.14 
0.88 
0.56 
1.40 
No 
T 
Period Before vs
AfterY1 
0.39 
0.28 
1.65 
1.22 
2.24 
Yes 
T 
Period Before vs
AfterY2 
0.39 
0.42 
0.88 
0.66 
1.18 
No 
T 
Period Before vs
AfterY3 
0.39 
0.30 
1.52 
1.12 
2.06 
Yes 
T 
Period AfterY1 vs
AfterY2 
0.28 
0.42 
1.86 
1.37 
2.54 
Yes 
T 
Period AfterY1 vs
AfterY3 
0.28 
0.30 
1.09 
0.79 
1.49 
No 
T 
Period AfterY2 vs
AfterY3 
0.42 
0.30 
0.58 
0.43 
0.79 
Yes 
NY 
Twolane 
Paved 
C 
Period Before vs
AfterY1 
0 
0.02 
infinity 
infinity 
3.18 
No 
C 
Period Before vs
AfterY2 
0 
0.12 
infinity 
infinity 
0.37 
Yes 
C 
Period Before vs
AfterY3 
0 
0.18 
infinity 
infinity 
0.23 
Yes 
C 
Period AfterY1 vs
AfterY2 
0.02 
0.12 
5.70 
1.44 
37.92 
Yes 
C 
Period AfterY1 vs
AfterY3 
0.02 
0.18 
9.07 
2.44 
58.83 
Yes 
C 
Period AfterY2 vs
AfterY3 
0.12 
0.18 
1.59 
0.67 
3.89 
No 
T 
Period Before vs
AfterY1 
0.38 
0.31 
2.79 
1.39 
5.84 
Yes 
T 
Period Before vs
AfterY2 
0.38 
0.27 
1.68 
0.88 
3.26 
No 
T 
Period Before vs
AfterY3 
0.38 
0.27 
1.72 
0.91 
3.30 
No 
T 
Period AfterY1 vs
AfterY2 
0.18 
0.27 
1.66 
0.78 
3.63 
No 
T 
Period AfterY1 vs
AfterY3 
0.18 
0.27 
1.62 
0.77 
3.52 
No 
T 
Period AfterY2 vs
AfterY3 
0.27 
0.27 
0.98 
0.49 
1.98 
No 
T = Treatment sites resurfaced with safety
edge.
C = Comparison sites resurfaced without
safety edge.
The odds ratio for the treatment sites was less than 1.0 for
one out of six cases, indicating that resurfacing
with the safety edge treatment is effective in reducing the proportion of
extreme dropoff heights. Resurfacing without the safety edge treatment
was effective in reducing the proportion of
extreme dropoff heights in four of six cases. Additionally, none of the
observed odds ratios less than 1.0 and almost all of the observed odds
ratios greater than 1.0 were statistically significant.
For after year 1 versus after year 2, an odds ratio point
estimate greater than 1.0 indicates that the second
year after resurfacing had more dropoff heights exceeding 2 inches than the
first year after resurfacing. Since there were more dropoff heights exceeding 2
inches in after year 2 as compared to after year 1 (10 of the 12
cases shown in table
8), there appears to be deterioration of the shoulder
condition in the second year after resurfacing. However, only about half of
these observed differences in the proportion of dropoff heights exceeding 2 inches
were statistically significant at the 5 percent significance level.
For after year 2 versus after
year 3, an odds ratio point estimate greater than 1.0 indicates that the third
year after resurfacing had more dropoff heights exceeding 2 inches than the
second year after resurfacing. Since 7 of the 12 cases shown in table
8 have an odds ratio point estimate of 1.0 (or nearly
1.0), which indicates no change in the proportion of dropoff heights exceeding
2 inches, there appears to be minimal
deterioration of the shoulder condition in the third year after resurfacing.
The before period dropoff height data were compared to the
after year 3 dropoff height data to
determine whether dropoff heights had increased to the levels that existed
before resurfacing. For this
comparison, an odds ratio point estimate less than 1.0 indicates that after
year 3 had more dropoff heights exceeding
2 inches than the period before resurfacing. Since the odds ratios were
greater than 1.0 in 7 of the 12 cases shown in table 8, there does not seem to be much evidence to suggest
the proportion of high dropoffs after year 3 differs from the before period.
A final comparison of
dropoff height data was made between sites resurfaced with and without the
safety edge treatment in the third year after resurfacing to determine if the
safety edge treatment has any role in development of dropoffs. The results of
this analysis are given in table
9. Odds ratio values
above 1.0 indicate that comparison sites had more dropoff heights exceeding 2 inches
than treatment sites.
Table 9. Comparison of the proportions of dropoff heights exceeding 2 inches between treatment and comparison sites for the final period after resurfacing.
State 
Road type 
Shoulder type 
Site type 
Dropoff heights that exceed 2 inches 
Odds ratio point estimate 
Lower 95% confidence limit 
Upper 95% confidence limit 
Statistically significant 
Number 
Proportion 
GA 
Multilane 
Paved 
C 
2 
0.02 
0.286 
0.040 
1.374 
No 
T 
5 
0.08 
Twolane 
Paved 
C 
6 
0.04 
1.034 
0.341 
2.922 
No 
T 
9 
0.04 
Unpaved 
C 
38 
0.08 
0.796 
0.476 
1.349 
No 
T 
27 
0.1 
IN 
Twolane 
Paved 
C 
21 
0.14 
10.332 
3.470 
44.394 
Yes 
T 
3 
0.02 
Unpaved 
C 
41 
0.14 
0.384 
0.256 
0.567 
Yes 
T 
112 
0.3 
NY 
Twolane 
Paved 
C 
10 
0.12 
0.382 
0.161 
0.858 
Yes 
T = Treatment sites resurfaced with safety
edge.
C
= Comparison sites resurfaced without safety edge.
The results in table
9 indicate that there were no differences in extreme
dropoffs between sites resurfaced with and without the safety edge in Georgia.
In Indiana, sites with paved shoulders resurfaced
with the safety edge had fewer dropoffs. However, sites with unpaved shoulders
showed the reverse trend. In New York, sites resurfaced without the safety edge
had fewer proportions of extreme dropoff heights. Taken together, these
results are inconclusive.
The analysis of the field measurements of dropoffheights
suggests that resurfacing is effective in reducing the proportion of extreme
dropoff heights. It also suggests that resurfacing with the safety edge
treatment does not increase the number of extreme dropoff heights and is
similar to resurfacing without the safety
edge treatment in reducing the proportion of extreme dropoff heights over
time.
