Tidbit Tuesday
Tuesday, September 15, 2020 at 4:32:26 PM

Traffic Study - To Be, or Not To Be

There has been some confusion as to what is or is not a traffic engineering study. Many people have said that in order to be a traffic study, it shall follow the Traffic Engineering Process and Report. This is not true. A traffic study can take on many forms depending on what is being studied and the purpose of the study. If you are trying to fix a safety or operations issue by determining changes in geometrics, capacity, traffic control or access, then you would follow the Traffic Engineering Process and Report. But studies such as no passing zones, pedestrian crossing studies (unless signal timing changes capacity), speed studies or queue analysis will not follow the Traffic Engineering Process and Report. All are considered engineering studies, and all will require an engineer to collect data, analyze data and make an engineering decision based on that data. All of these engineering “reports” require an engineer’s seal. I use reports loosely because it could be only one page or many pages.

There are many different types of traffic studies that range from small-scale to large-scale no matter what process they follow. Don’t assume what you did on one study will be what is required on another study. These studies are not recipes in a cookbook; it depends on purpose and need, context and limitations as to what may be required. The guidelines for traffic studies can all be scaled to fit the context of the assumed problems. The bones of all studies are the same: collect data (or use existing data if it exists), evaluate existing conditions, determine issues (develop problem statement), and evaluate alternates based on the problem statement and your limitations.

Let me give you an example that can go from a very small study to something very complicated: A new pedestrian crossing at a signal. The first thing you would do is look at existing conditions: take signal timings for each “plan” that runs during the week (don’t look at every plan programmed since some of these never run or only run during an emergency such as an evacuation), evaluate if timing exists such that a pedestrian can cross according to MUTCD requirements, and perform a geometric field check to determine if the crossing is ADA compliant. If existing signal timings for all day allow pedestrians to cross the street without any changes and the crossing is ADA compliant—the study is complete and the recommendation would be to install the proper signal equipment. But if the crossing is not ADA compliant, then the study should lead to a construction project to build the necessary pedestrian facilities to be ADA compliant.

If the engineer determined the timings are not adequate for pedestrians to cross, then further study of the signal will be needed. This typically (not always) means determining the geometric features needed to allow pedestrians to cross without reducing the capacity of the signal. Alternatives that may be looked at would be: 1) pedestrian refuges if it’s a multilane crossing to try and shorten the time/distance needed for pedestrians to cross so as not to affect existing signal timings, or 2) lengthening turn lanes to provide for more storage due to the extended timings necessary for pedestrian crossings. If geometric changes can’t mitigate the crossing distance, and this is not an isolated signal but is located in a coordinated signal system, then other signals may be involved in the study. If cycle lengths or phasing changes are needed then your study just expanded to include the entire coordinated signal system. This means that data would need to be collected at all signals within the system and a signal timing study would have to occur. So something that could be simple can turn quite complicated depending on existing conditions. Everything is dependent on the existing conditions, the context of the area, and the assumed issues.

When tackling a study, you should always start small and let the data lead you into the direction you need to go. Don’t start with the Taj Mahal - start with the modest 2 bedroom/1 bath cabin and grow from there. If you start with the complex, then you increase the risk your study will get bogged down. Remember, you started this study because someone thought there was something to fix. If we spend all of our time studying things that probably won’t happen, then that perceived issue never gets addressed.

If you'd like to contact us with any comments, questions or tidbits of your own, please email TrafficEngineering@la.gov.

Thanks for reading!