1. SUMMARY AND CONCLUSIONS
SAFETY OF A SPECIAL TIME-OF-DAY PROTECTED/PERMITTED LEFT-TURN SIGNAL CONTROL DISPLAY
Ozlem Ozmen, University of Nevada, Reno
Zong Tian, University of Nevada, Reno Reed Gibby, Nevada Department of Transportation, Carson City
CRASH DATA ANALYSIS
Analysis Based on Crash Rate
The left-turn crash rate (in crashes per million entering vehicles, C/MEV) was calculated for each study approach using Bonneson’s equation.
ABSTRACT
Table 1 Leading Left Turns for Standard MUTCD and Las Vegas LT
Analysis Based on the Simple Before-after Study Method
All Sites
Estimate λ and predict π
The estimated total crashes in the “after” period λ = L = 37
The predicted total crashes in the “after” period π = K = 21
Estimate the variance and standard deviation of π, VAR(π ) and σπ
VAR(π) = K = 21. σ π = sqrt(21) = 4.58.
Estimate δ = π – λ = 21 – 37 = -16.
This indicates an increase of 16 crashes for all the study sites over a two-year period.
Estimate the confidence interval
At a 95% significance level (α = 0.05), the confidence interval is estimated at
(π – 2σπ , π + 2σπ ) = (11.84, 30.2).
Since L = 37 > 30.2, the crash increase was statistically significant.
Excluding One Unusual Site
The intersection of Charleston/Las Vegas had an unusual crash increase compared to the other sites, so this site was considered as an outlier and was excluded from the following analysis.
The estimated total crashes in the “after” period λ = L = 24
The predicted total crashes in the “after” period π = K = 19
Estimate the variance and standard deviation of π, VAR(π) and σπ
VAR(π) = K = 19. σπ = sqrt(19) = 4.36
Estimate δ = π – λ = 19 – 24 = -5.
This indicates an increase of 5 crashes for all the study sites over a two-year period.
At a 95% significance level (α = 0.05), the confidence interval is estimated at (π – 2σπ , π + 2σπ ) = (10.28, 27.72).
Since < L < 27.72, the crash increase was not statistically significant—in fact, it is a significant reduction.
Analysis Based on the Before-after Study with Comparison Group Method
The cross-street approaches of each study site that use standard MUTCD left-turn displays were selected as comparison sites, and only crashes occurring during Las Vegas LT Display time-of-day operation were used for the analyses.
The estimated crashes in the “after” period for the treatment group, λ = L = 37
rT = rC = (N/M)/(1+1/M) = (29/25)/(1+1/25) = 1.12
π = rT*K = 1.12*21 = 23.5
Estimate VAR(π) and VAR(ω)
VAR(ω) = s2(O) – (1/K + 1/L + 1/M + 1/N)
Where s2(O) is the sample variance of the Odds Ratio, O, which is calculated by
O = (KN)/(LM)/(1+1/L+1/M) for each year,
VAR(ω) = 1.72 – (1/21 + 1/37 + 1/25 + 1/29) = 1.57
VAR(π) ≈ π2[1/K + 1/M + 1/N + VAR(ω)]
= 23.52[1/21+1/25+1/ ]= 934.5
σπ = sqrt[VAR(π)] = 30.6.
Estimate δ = π – λ = 23.5 – 37 = -13.5, this indicates an increase of about 14 crashes for all the study sites over a two-year period.
At 95% significance level (α = 0.05), the confidence interval is estimated at
(π – 2σπ , π + 2σπ ) = (-37.7, 84.7).
Since < L < 84.7, the crash increase was not statistically significant.
This paper provides a safety evaluation of a special protected/permitted left turn signal control that has been implemented in the urbanized area of Las Vegas, Nevada. The special left turn display provides protected only left turns during certain times of day by suppressing the permitted green ball and yellow ball displays. During other time periods, the signal resumes standard five-section protected/permitted operations. This special operation allows the use of lead-lag phasing during protected only control to improve progression and standard protected/permitted control for improving capacity and minimizing delays during low-volume time periods. Before and after studies were conducted using the crash data from 10 intersections that have implemented the special left-turn display. Results from the analyses indicated that there is no apparent increase in crashes, thus indicating no obvious safety concerns due to use of the special display.
The primary objective of this paper is to assess whether the Las Vegas LT Display results in any safety concerns through examination of before and after crash records.
Table 4 Data for Crash Rate Calculations
SUMMARY AND CONCLUSIONS
Table 2 Time-of-Day Operation of the Las Vegas LT Display at the Study Intersections
This paper documented the safety performance for the Las Vegas LT display. Crash statistics from ten intersections that implemented the Las Vegas LT display were used for the analyses. Crash statistics included a two-year before period and a two-year after period. Several before-after study methods were used to draw conclusions from both statistical and practical point of view. The analysis methods included the simple before-after study, before-after study with comparison group, and calculation of left-turn crash rates. Based on the results from the analyses, the following findings and conclusions were reached regarding the safety aspects of the Las Vegas LT display:
Findings:
The study intersections involved a small number of crashes before and after installation of the Las Vegas LT display. One reason was that the Las Vegas LT Display only operated during a small portion of a day, typically the peak-periods. As a result the number of crashes was very small, which made it difficult to obtain statistically valid results. With this limitation in mind, the various analyses did not find a significant increase in crashes when the Las Vegas LT Display was in operation.
The traffic volume data were available only at six intersections where the left-turn crash rates could be calculated. These six intersections yielded an average of 0.45 crashes per million entering vehicles. This average crash rate was the lowest compared to the crash rates reported in previous studies in which the MUTCD compliant left-turn displays were used.
Conclusions:
Based on the results from various analysis methods, it can be concluded that the Las Vegas LT Display did not reveal any obvious safety concerns when compared to the MUTCD standard PPLT control.
To increase the sample size, it would require crash statistics be collected over a longer period of time. In addition, alternative evaluation tools may be beneficial to truly evaluate the safety performance of the Las Vegas LT display. For example, instead of totally relying on crash statistics which are highly random in nature, surveying of drivers’ understanding of the Las Vegas LT Display or conducting field observations of drivers’ behavior would provide additional useful information. This would justify a further study related to evaluation of safety of the Las Vegas LT Display.
INTRODUCTION
Protected/permitted left turn (PPLT) control has the advantages of both permitted only and protected only left turn controls by increasing left turn capacity and reducing delay at intersections. The protected left turn phase either precedes (leads) or follows (lags) the opposing through movement. The major operational concern for PPLT is the so-called “yellow-trap” condition if lead/lag phasing is used. A yellow-trap occurs when a permitted left turn phase is terminated during the change from permitted left turns in both directions to a lagging protected left turn in the opposite direction.
Several transportation agencies in the United States have designed and implemented unique PPLT phasing arrangements and displays to eliminate the yellow-trap condition. Recognizing the limitations of the standard MUTCD PPLT display, the City of Las Vegas has deployed a unique PPLT display (referred to as the Las Vegas LT Display) to eliminate the yellow trap condition and improve progression during the peak periods. The Las Vegas LT Display provides protected only left turns during the peak periods of day by suppressing the permitted green ball and yellow ball indications. During other time periods, the signal resumes standard five-section protected/permitted operations. Internal signal controller logic is used to implement protected only left turn control during certain time-of-day operations.
Table 3 Intersection Crash Data for the Subject and the Opposing Approaches Using Two-Year Before-After Analysis Periods
Figure 1 Las Vegas LT Display-Louvered Red Ball Following the Yellow Arrow