To Pass or Not to Pass?
Analyze Unlimited Miles for (Much) Less And Save Lives
Introduction
We’ve all experienced such passing anxiety, and whether we realize it or not, when faced with the decision to pass we are essentially stressing over the question of how accurate the passing zone markings are. Did they get it right?
With human lives at stake, transportation agencies such as the Utah Department of Transportation (UDOT) concern themselves with this question on an onging basis as they go through their multi-year cycles of refurbishing, repaving, and restriping their roads.
Conventional Passing Zone Studies
According to the National Highway Transportation Safety Administration (NHTSA) approximately 95% of US car accidents are caused by human error.
DOTs are continually spending money to restripe their roads as part of their regular multi-year road maintenance cycles. Restriping can include Passing Sight Distance (PSD) studies, particularly in areas where crashes and incidents have been occuring more frequently. Considering the fact that passing a car on a two-lane highway is highly dependent on human factors, we might expect that the methods for determining where it is safe to pass, i.e., passing zone studies, would employ the most recent techniques and technologies. Regretfully, this isn’t the case.
How It’s Done Today?
Present day passing zone studies are usually carried out by two vehicles driving in tandem at a pre-calculated distance deemed safe for passing. The trailing car represents the passing car, and the lead car represents oncoming traffic. The cars travel along the entire route in question and areas where the lead car is visible to the trailing car are considered safe for passing. Said another way by a reputable engineering firm, the common method of conducting passing zone was described as follows: “Currently, the established method is to estimate passing zones by driving the corridor with two vehicles to determine where passing sight distance is available and where it is not.”

The Challenges
As you can imagine, this method has many challenges, both from a safety perspective and from a DOT’s economic perspective. Some of the challenges include:
Labor – Current models require you to physically traverse each mile in the study.
Cost – Conventional passing zone studies are charged by the mile, and with human involvement, can be quite costly.
Limited Scope – Because of the expensive and limited resources involved, it is not economical to do such studies across all road sections. While many sections may not require studies, it is likely that amongst the thousands of miles under a DOT’s responsibility, some sections that should be analyzed will be passed over and some other basis will be used for striping, such as maintaining the existing striping.
Rapid Obsolescence – Passing zone studies can become obsolete because of changes such as increased speed limits, changes in foliage, commercial growth, etc.
Reactive – DOTs usually run passing zone studies where crashes have already occurred, rather than using an proactive approach that predicts where crashes are likely to occur.
Inflexibility – The analysis is conducted using certain parameters, and it isn’t practical to redo a test with a different set of starting parameters or to run the analysis multiple times.
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A New Approach to an Old Problem
“The passing zones as determined by RDV do line up with the passing zones identified by the study.”
From Evaluation Report

“…using the LiDAR data may provide a more precise analysis and may result in less man-hours being used to complete a study.”
From Evaluation Report
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