Mike Spack and Bryant Ficek have literally written a book about Traffic Impact Studies and the process from authorization to final study. We present the information we wish we had when starting our careers and hopefully have tips and refreshers that even experienced traffic engineers will find useful. This series presents the basic steps of completing a Traffic Impact Study from their book. See the early posts here.
Existing traffic data is the base for your analysis and the foundation for forecasting future traffic volumes. Your purpose in this task is to, therefore, gather or collect up-to-date and accurate traffic data. For most studies, obtaining the daily traffic volumes and peak hour turning movement volumes is sufficient.
The Average Daily Traffic (or ADT) is the total volume on the road in both directions over a typical day. ADTs can provide insight into the number of lanes needed on a corridor.
The peak hour traffic is usually obtained at an intersection and documents the number of vehicles making each movement through an intersection – lefts, throughs and rights – for a single hour from every direction. Peak hour turning movement volumes are the foundation for most traffic analyses performed as intersections are the pinch points in a roadway network.
Focusing on the daily and peak hour turning movement volumes, the four ways to get existing data are:
The easiest way to find traffic data is by going to the websites of the city, county and state where your study is located. In Minnesota, government agencies are required to count certain roads every two or four years. The current count data for those roads are available online and are very accessible. We both have a lot of bookmarked websites that get us quickly to different agency datasets. We most frequently visit the Minnesota Department of Transportation’s (MnDOT’s) site, which has both existing and historical daily traffic volumes going back ten years for all state roads as well as many city and county roads.
Another option for finding traffic data is digging for recent traffic studies that cover your study area. As we mentioned, almost all traffic studies have existing data and traffic forecasts. Finding a traffic study done by a government agency or developer that covers the same roads or intersections will provide you a lot of insight about the study area as well as the traffic study format the public agencies prefer.
Be aware of the limitations of finding traffic volumes or other studies online, especially the age of the information. Your analysis and forecasting should be based on existing traffic, not on something too outdated. We recommend using daily traffic volumes less than two years old and turning movement counts less than a year old.
If you do find current daily and peak hour traffic volumes for the locations you need, consider yourself very lucky and save it in your files immediately. If you don’t find the exact data you need, historical and surrounding area data can also be useful and should be downloaded.
Your next options are to collect the traffic data yourself internally or to hire a sub-consultant externally to complete the data collection. The process for both options is similar, so we tackle them together in this section.
There are pluses and minuses to both internal and external counting. Generally, we recommend outsourcing the data collection if (1) there is a trustworthy company you can outsource the counts to who can meet your deadline/budget and (2) you don’t already have the gear/people available to do the counts. Your study likely planned for internal or external counts in the budget, so this decision should have already been made by you or your boss.
Whether you or someone else will be doing the actual traffic data collection, you should get your traffic counts scheduled immediately. Internally or externally, your counts are not the only thing going on. The sooner the count is on the board, the sooner you get the numbers for your analysis.
Because collecting data with video cameras have become inexpensive, we’ve defaulted to collecting 48-hour turning movement counts. Why 48-hours? Because it ensures we have the various peaks during the day (typically morning, noon, and afternoon peak hours) regardless of when they occur. We also then have good data if we need to perform a warrant analysis that requires at least eight hours of data. Other good reasons for 48-hours are providing daily volumes for the study corridors, averaging the counts to smooth minor irregularities, and avoiding major irregularities if an event occurs during your count (police action, crash, spontaneous parade, etc.).
Also as a default, we count heavy vehicles, bicycle movements, and pedestrian movements. Knowing this extra data can assist with your overall site review and multi-modal analysis.
Other key points to remember for your data collection:
Scheduling or ordering your data collection doesn’t mean you’re locked into your data collection plan. Ask your technician or sub-contractor when they plan to collect the data and the last day you’ll be able to make any tweaks to the data collection plan. Be sure to make any necessary changes by that last day. Although any sub-contractor or technician will likely be happy to go back to the site for additional data, they won’t be doing it for free.
Please take safety seriously. No traffic count is worth an injury and you should consistently tell all field staff to be cautious, no matter what kind of field work they’re doing. Never step into a road without looking both ways first; this is a habit that should be developed by all field personnel. Be very careful where you park vehicles. Intersection counting is sometimes necessary in early mornings or late evenings when it’s dark. Try to locate yourself in well-lit areas where other drivers can see you. Having a cell phone to call for help if needed is also a great idea. If a count is needed in a high crime area, be sure to have at least two people per vehicle.
Be sure to use all the typical safety gear (including reflective vests, jackets, and hats) and ensure your gear complies with relevant safety standards. An orange tee-shirt isn’t nearly enough. You should have flashing beacons on your work trucks and vans, which are used whenever you stop for installation on the road. Also set out cones around your work area, but remember that you likely don’t have authority to close a lane and you could have serious liability if a motorist is hurt by something you did.
Flashers on a truck can give you and your co-workers a false sense of security, when, in fact, flashers won’t physically stop a distracted or impaired driver from running into a vehicle. There are many YouTube videos available showing drunk drivers running into parked police cars that have their flashers on, so be cautious.
The bottom line is to take the necessary precautions for any field work. Be aware of your surroundings and trust your instincts. If you feel unsafe in an area, leave and come back the next day with more help.
Estimating traffic volume data can be as simple as saying our study has less traffic than the adjacent collector street because:
Therefore, it’s logical to assume the study road has the same amount of traffic or less. Since the two-lane collector road has a capacity of at least 10,000 cars per day, it’s safe to assume the daily volume on the study road is well below its capacity. If you’re only adding a modest amount of daily traffic, no further study is needed. “Modest” is a judgment call though. If you’re adding thousands of vehicles per day to a road or if you think the road’s traffic volumes may be more than 75% of its capacity, estimation isn’t appropriate.
An often-used traffic engineering guide is that the daily traffic on a road is ten times larger than the peak hour traffic on the road. This relationship is corroborated in Table 41 of NCHRP Report 365 – Travel Estimation Techniques for Urban Planning. Another way to think of this is that the peak hour traffic is about 10% of the daily traffic.
In Minnesota and other states, permanent Automatic Traffic Recorders (ATRs) are embedded in the road to record traffic 365 days a year, 24 hours a day. This data is often posted to the DOT’s website, but it’s usually buried quite deeply. It’s worth finding out what type of ATR data is available in your region, because the ATR data can help you refine your peak hour to daily traffic ratio general guidance – maybe it’s 9% or 11%.
Assuming you have the p.m. peak hour turning movement count at an intersection, this guide can help you extrapolate daily traffic volumes. Add up the entering and exiting volume on one leg of the intersection and multiply the sum by ten. Besides a decent estimate of daily traffic volume, this method can also be used as a quality control check. Ensure the peak hour turning movement counts and the daily traffic volume data you have are close to the 10% benchmark. You may have bad data if your peak hour volume is 3% or 30% of the daily volume.
Sometimes the city will set up its temporary mechanical tube counters to collect hourly and directional data when it’s collecting daily traffic volumes. If you get data refined to this level and assume a directional orientation, you can break the directional peak hour count into a turning movement count. This method can work out at a low-volume, rural tee intersection, but this method isn’t appropriate for analyzing a busy suburban intersection.
If you have a p.m. peak hour turning movement count, you could reverse the numbers to approximate the a.m. peak hour turning movement count. Typically, the a.m. peak period has lower traffic than the p.m., so this likely results in a conservative analysis. Reversing volumes works in areas where there is a heavy commuter pattern. Reversing the p.m. peak hour data to represent the a.m. peak hour wouldn’t be appropriate in a shopping district, for example.
Keep in mind that estimating traffic is not the same as obtaining actual traffic counts. Individual intersection and corridor volumes can vary greater from the general guidelines presented here. We strongly recommend you get the actual traffic counts. You open yourself up to potential questions and criticism if you only use estimated traffic as the base for your study.
Whether your study needs these or other types of traffic data should be determined before you start the study and listed in your scope of work. For a typical TIS, you won’t need specialized types of data like gap studies or queue counts. For any data collection needs, always remember to think your way through the process, preferably in the proposal stage. Know what you need and how you plan to get it. Besides helping you stay on schedule and on budget, that knowledge always makes for a smoother project.
This is a multipart series. Check out the other articles in the series.
Did you like this discussion regarding Traffic Impact Studies?
Our Traffic Studies Manual contains more details, checklists, and real-life situations Mike and Bryant have dealt with on their projects. Also check out our Traffic Impact Study Report Template. This is the template we use when creating reports and is completely editable in MS Word.
At Spack Academy you can also find other guides, case studies, research briefs, and spreadsheet tools that help us be better traffic engineers. Visit us at www.SpackAcademy.com, connect with us on LinkedIn, or send us an email with your thoughts and questions.