Evaluation of the Effectiveness of Converging Chevron Pavement Markings

Investigators:

Dr. Michael Hunter, Associate Professor, Georgia Tech
Dr. Angshuman Guin, Research Engineer II, Georgia Tech
Saroch Boonsiripant, Graduate Student, Georgia Tech
Dr. Michael Rodgers, Principal Research Scientist, Georgia Tech

Project Overview:

The project focused on an investigation into the effectiveness of chevron markings in reducing vehicle speeds on two-lane freeway-to-freeway directional ramps in a desire to improve safety performance.  The evaluation is based on a statistical comparison of speeds before and after the installation of the chevron markings at pre-selected sites in the Atlanta, GA area.  The analysis focuses on the impact of converging chevrons over the range of speed percentiles and on the mean speed through the use of diverse sampling and control ramps

Study Sites

The evaluation of the effectiveness of converging chevron pavement markings has been performed at two interchange locations in Atlanta, Georgia: Interstate 75/85 interchange and Interstate 75/285 interchange.  For each site two ramps were selected - a treatment ramp, i.e. the ramp containing the chevron installation, and a control ramp, i.e. a ramp with no treatment applied, to monitor potential underlying changes in traffic operations at the interchange. Data collection stations are placed immediately upstream and downstream of the chevron marking location.

Data Description

Streaming Per-Vehicle Record (PVR) data and binned vehicle speed data was recorded in the field and periodically downloaded for analysis. The PVR record data for each vehicle contained parameters including time stamp, lane number, vehicle class, vehicle speed, vehicle length, time headway, and distance between axles. The data were collected between March 2008 and February 2009.

Findings

Analysis of the speed data indicates that the presence of the chevrons had only a modest impact on overall vehicle speeds.  The observed changes in the speed distributions are reasonably consistent between the test sites thus increasing confidence in the findings.  The effect of the treatments was most pronounced immediately following implementation, with the impact waning over the duration of the study.  By the ninth month the magnitude of the impact was under 2 mph over most of the vehicle speed percentiles.

To account for sampling differences and potential distributional difference between the data collected over the different time periods a “Monte Carlo” random sampling strategy of lead vehicles was adopted for the analysis.  The results of this analysis showed an average speed reduction upon entering the controlling ramp geometry on the order of 0.5 to 2.0 mph by the ninth month after treatment, with much of this effect related to increased speeds on the control ramp.  In addition, it is noted that there was minimal effect on the speed variance of vehicles entering the controlling ramp geometry.  When considering the potential for speed reductions between the detectors it would appear the chevrons had little to no impact, with the control ramps experiencing similar trends as the treatment ramps.  Taken together this would imply the mean effect of the chevrons is limited to a 0.5 mph to 2.0 mph reduction in mean speed as the vehicles enter the controlling ramp geometry.  

Thus, it was found that the impact of the chevron treatment on speed tends to be minimal, with drivers adjusting back to their previous speeds as they acclimate to the treatment. However, this does not necessarily indicate that the chevrons are not a meaningful safety treatment but rather that any safety benefits are likely not due to a general decrease in speeds.  For example, it is possible that the chevron treatment may help alert an inattentive driver thus reducing the likelihood of an accident occurrence without having a significant impact on the overall population.  Additionally, an incident analysis for the subject ramps showed that a significant subset of the crashes occurred under wet or snowy conditions and it is possible that the chevrons function differently under these adverse conditions.  Data collection constraints precluded examination of this possibility. Prior to any final judgment on the effectiveness of the chevron treatment on safety it is recommended that a direct accident study (as opposed to utilizing a surrogate such as speed) be conducted after sufficient accident data has been gathered. It is also noted that the minimal speed reduction is based on two two-lane freeway ramp sites in one geographic area and additional research is needed. 

Sponsor: Georgia Department of Transportation