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LANE ENCROACHMENTS

AWP has been proven to be more retroreflective than standard pavement marking; however, the researchers found no conclusive evidence that speeds increased or decreased according to the type of pavement marking used. Taking a more direct approach, lane encroachments were used to evaluate the two pavement marking types. Lane encroachments are defined as vehicles that infringe upon the adjacent lane by crossing the center line striping of a multi-lane highway.

The research team examined lane encroachments at the North Carolina test sites only, as they had temporary lane shift detours installed during bridge construction projects.

Data Collection

Lane encroachment data were obtained through manual video extraction for two test sites in North Carolina. Once temporary detours (em.e., lane shifts) had been fully constructed and delineated with work zone pavement markings, the team set up video cameras to record highway activities at entering and exiting curves into and out of lane shifts. Video was recorded to a DVR from a high elevation to give the observer a clear overhead view of the lanes and lane lines. The team focused primarily on capturing rainy nighttime conditions for at least 2-hour periods. When sufficient video was recorded, the team took the equipment back to the lab for investigation and data extraction.

From the video, traffic volumes and lane encroachments were collected either for a total count or at 5-minute intervals. Traffic volumes were obtained simply by counting the number of vehicles passing a pre-designated segment of highway, whereas lane encroachments required additional effort. During nighttime conditions, lane lines were difficult to make out in the darkness of the videos. Therefore, the observer physically traced pavement striping from daytime footage and used these traces as lane line references for the nighttime footage. Lane encroachments were counted when a vehicle’s tire crossed over a solid pavement stripe. Additionally, only the lead vehicle of a platoon was observed so as not to include vehicles simply following the lead vehicle’s taillights.

Statistical Analysis of Lane Encroachments

I-85

This site was a single-lane crossover with barriers and RPMs; therefore, lane encroachment analysis was not conducted due to the quality of video and fact that this particular site was only a single lane (and thus had no second lane to encroach upon).

US-15/501

Multiple videos were recorded at a work zone crossover in 2009. Videos of nighttime, rainy conditions were shot on May 28 and July 12 between 11:00PM and 1:00AM and 9:00PM and 12:15AM, respectively. Additionally, the rainfall intensity was approximately 0.5 inches (12.7 mm) per hour for both recordings. Total vehicle counts and the number of vehicles that encroached lane lines were tallied. Although other video of nighttime rainy events were available, the lack of rain intensity and duration limited their use.

During the May video recording, standard pavement markings were installed. There were a total of 824 vehicles, of which 36 encroached. By the time of the July recording, the AWP markings had already been laid over the standard pavement markings. The AWP data had a total of 1,493 vehicles recorded, of which 30 encroached. Table 11 displays the summary table of lane encroachments.

Table 11. Chi-square results for US-15/501 lane encroachments.

Pavement Markings Encroachments Non Encroachments Total
Standard 36 788 824
AWP 30 1463 1493
Total 66 2251 2317
    Chi Square (X2) 10.68203

The percentages of vehicle encroachments were 4.6 percent and 2.1 percent for standard pavement marking and AWP markings, respectively. A Chi-square test indicated that the difference in lane encroachments for the AWP and standard pavement marking was statistically significant at the 99 percent confidence level. Therefore, the AWP appears to have reduced the percentage of lane encroachments at this crossover.

US-421

As noted earlier, at the US-421 test site there were lane shifts in both directions of the divided highway, both having very similar geometry. The westbound direction was treated with standard pavement marking while the eastbound direction was treated with the AWP. For each direction of travel, there were two crossovers (entry and exit). Each crossover contained entering and exiting curves. Therefore, at the US-421 site, there were a total of eight curves in which lane encroachments could occur—two per crossover, or four per direction. Lane encroachments were only taken for the lead vehicle of a platoon.

The team obtained 3.5 hours of video during rainy nighttime conditions concurrently at all of the curves. The actual vehicle counts were 2,582 for standard pavement marking (westbound direction) and 1,411 for the AWP (eastbound direction). There were 270 and 166 lead vehicle lane encroachments, respectively. Foreseeing the variability of traffic conditions, the observers recorded vehicle counts and lead vehicle encroachments at 5-minute intervals. Thus, 42 5-minute increments were obtained for nighttime, rainy conditions at each of the 4 crossovers.

Using the 5-minute samples of traffic volumes and lead vehicle encroachments, lane encroachment rates were calculated. The mean and standard deviations were computed from the samples of lane encroachment rates. Samples were compared, and their mean differences determined. Finally, t-tests were used to analyze the significance of various sample comparisons. The results of the t-tests were based on a 95 percent confidence interval.

When analyzing the lane encroachment rates, comparisons were made between standard pavement marking and the AWP. The comparisons were as follows:

  • Standard pavement marking entry versus the AWP at an entry crossover.
  • Standard pavement marking exit versus the AWP at an exit crossover.
  • AWP at an entry versus an exit crossover.
  • Standard pavement marking at an entry versus an exit crossover.
  • Combined lane encroachment rates of standard pavement marking entry and exit versus rates of the AWP at entry and exit crossovers.

Table 12 summarizes the findings of the statistical tests.

Table 12. US-421 lane encroachment statistics.

Statistical Analysis of Lane Encroachment Rates

Sample Comparison Mean Standard Deviation Mean Difference Two-tail p-value

Standard Entry

4.1%

3.2%

+1.2%

0.1935

AWP Entry

5.3%

4.9%

         

Standard Exit

6.4%

3.5%

+0.1%

0.8819

AWP Exit

6.5%

4.8%

         

Standard Entry

4.1%

3.2%

+2.3%

0.0028*

Standard Exit

6.4%

3.5%

         

AWP Entry

5.3%

4.9%

+1.2%

0.2591

AWP Exit

6.5%

4.8%

         

Standard Entry + Exit

10.5%

5.4%

+1.3%

0.3571

AWP Entry + Exit

11.8%

7.5%

* p-value is statistically significant (α = 0.05)

Table 12 shows that crossovers marked with standard pavement marking had slightly lower encroachment rates than similar crossovers marked with the AWP. However, the differences were small, and based on the results from the t-tests no lane encroachment rate comparisons, except between standard pavement marking entry versus standard pavement marking exit, had statistically different rates.

Conclusion and Recommendations

In North Carolina, the research team used lane encroachments as the primary factor to evaluate the safety measure of installing the brighter AWP. It was hypothesized that bright lane markings would enhance the driver’s ability to clearly distinguish lanes on the roadway and make safer driving maneuvers. The results of the lane encroachment analysis provided inconclusive findings.

First, when studying the lane encroachments at the US-15/501 site, the results from the experimental test show that the installation of the AWP reduced the rate of lane encroachments by 50 percent. That is to say, the percentage of vehicles that encroached out of the total number of vehicles was half that of the percentage calculated for standard pavement markings. Upon first glance, this is impressive, yet it should be interpreted with caution. A variety of confounding issues may exist, especially regarding the variable nature of rain intensity. However, studying the rate of lane encroachments at the US-421 site, the team sought to compare differences between entry and exit curves installed with the AWP and standard pavement markings. T-tests showed that the small difference in lane encroachment rates were statistically insignificant.

Another interesting finding is the lane encroachment rates between entry and exit curves using the same pavement markings, which indicate that drivers are likely making more lane encroachments at the exit curves. Recalling findings from the speed analysis, there were similar statistically significant differences between entry and exit curves. Mean speeds were higher in exiting curves than in entry curves. The type of crossover appears to be more important to traffic operations than other factors studied.



Page last modified on June 3, 2013.
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