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.

As you likely learned from your earlier discussions with the governing agencies, they usually have standards for acceptable traffic operations. If not, you can assume LOS E and F are unacceptable, which is common in most regions of the United States. Another measure could be the downgrade or worsening operations by either a LOS letter grade or a significant increase in delays or vehicle queues.

If you have an intersection that’s operating unacceptably in one of your study scenarios, it’s up to you to figure out how to make all that traffic work efficiently.

In simple terms:

1. Start with the earliest scenario that has an identifiable problem (existing before no build, before build, before future scenarios, etc.).
2. Select a potential improvement.
3. Run the capacity analyses with this improvement coded into your capacity analysis software.

Keep running improvement options until you hone in on solving the problem. Then once you’ve reanalyzed all your scenarios with acceptable results, document the improvements suggested and the timeline, or scenario, for when they’re needed. Let’s explore each step in more depth.

Identify Areas of Concern

The first task is to quantify when expected issues arise and the extents of the problems. There is obviously a huge difference between a failing right turn lane in the 20-year Build scenario and issues with the through lanes starting in the existing scenario. The easiest way to review everything is to write it all down. Grab yourself some paper and create a list. Starting with your existing scenario, list each intersection and movement where your results show unacceptable traffic operations. Another item to add to your list would be the volume-to-capacity or v/c ratio. As we discussed, a v/c ratio of 1.0 suggests “at capacity.” Above 1.0 indicates a facility is over-capacity, while below 1.0 suggests an under capacity facility. The v/c ratio will also help you determine the magnitude of any issues. If there are no issues in this first scenario, write “none.” Then proceed through your No Build scenario(s) and finally through your Build scenario(s).

With this list, you can now visualize where issues start, whether concerns are limited to one intersection or stretch through a corridor and whether the results are barely unacceptable – just over the acceptable standard- or really, massively bad, such as a v/c ratio that’s double the standard.

Determine Initial Mitigation

Now comes the fun part of determining solutions to the issues you identified, also known as mitigation. We usually examine supply-side mitigation first. Supply-side mitigation looks at changes to the roadway system to either accommodate more traffic or to make the traffic move more efficiently. You can use multiple supply-side improvements, including:

1. Signal timing adjustments
2. Traffic signal phasing changes, such as adding left turn arrows
3. Improved signing and pavement markings
4. Peak hour turning restrictions
5. Traffic control change, such as adding stop signs
6. Addition of an exclusive turn lane
7. Addition of a through lane
8. Alternative intersection traffic control
9. Access management options
10. Other roadway operational changes (change to one-way?)
11. Intelligent Transportation Systems (ITS) options

An often-overlooked solution is demand-side mitigation. While the supply-side focuses on the roadway system and increasing capacity, the demand side focuses on reducing traffic volumes. Demand-side mitigation suggests methods to reduce a development’s expected traffic generation, such as:

1. Pay for parking
2. Peak hour parking restrictions
3. Truck/delivery peak hour restrictions
4. Staggered work hours or days
5. Smaller development size

Stuck on ideas? Consider the comprehensive or transportation plan for the city or county. They may have already explored and suggested improvements that you can now confirm the need and set to a timeframe. Similarly, other recent traffic and transportation impact studies in the area may have identified future improvements.

As you think about the possible improvements for your situation, you’ll want to keep several key factors in mind – safety, public right-of-way, and feasibility. Considering these will help you decide which mitigation is best for your situation. Safety is obviously important since good operations won’t matter if drivers feel unsafe. An example would be providing a tunnel under the road to remove pedestrians from the vehicle flow. In theory, it may remove some conflicts, which would improve vehicular traffic flow. However, people simply won’t use it if the topography doesn’t lend itself to a tunnel or if the lighting and security are in question. Instead, you may have actually made the situation worse as pedestrians will continue to try to cross the road without the previous aid of the signal or other crossing assistance.

The right-of-way is the government-owned land available to accommodate improvements. This could include the actual roadway right-of-way or even the opportunity for acquiring right-of-way. An urban area may have buildings close to the right-of-way limits, reducing any options for increasing the right-of-way without demolition. It’s extremely rare to knock down buildings in a city to add a turn lane, nor good practice. This is a big reason roundabout projects are more popular in the suburbs and on the edge of town versus in the old core city. There just isn’t space available to add them inside older cities.

Feasibility measures could include local awareness and knowledge, the potential for ongoing maintenance or upfront cost. A dual-lane roundabout may be a great solution for reducing traffic delay and improving safety, but in an area without even a single lane roundabout, is it the most appropriate option for you?

Similarly, there could be an ITS option that would improve safety and operations at a stop-controlled intersection. But if the local government doesn’t have the proper personnel to maintain it, another mitigation strategy is probably best.

Consider continuity through your solutions as well. For instance, a right turn lane used as mitigation today could also be part of an additional through lane in the future to reduce those future costs.

Your local, county or state government may also have some warrants or general guidelines contained in their standards that you can use in determining appropriate mitigation. Some general ideas include:

• Use an exclusive turn lane if the turning volume exceeds 100 vehicles per hour.
• Use dual left turn lanes if the turning volume exceeds 300 vehicles per hour.
• Use the signal warrants from the Manual on Uniform Traffic Control Devices (MUTCD).
• If a signal is warranted, a roundabout could also be considered.
• If all-way stop control is warranted, a roundabout could also be considered.

Beyond those basics, any mitigation is up to you and will depend on your exact situation. Try to use these general guidelines to think through and develop the best mitigation option. If you are just starting out on your first traffic impact study, you may want to take your list of traffic operation issues to your supervisor so you can both determine how to proceed with potential mitigation.

As you gain experience, you’ll quickly get better at developing the initial list of possible mitigation strategies. Start with the cheapest options – adding stop signs or changing signal timing. From there, examine changing the phasing at the signal to see if something like going from protected to protected/permitted left turn phasing might help. The next level is looking at adding signals, roundabouts or turn lanes. Finally, your big capacity improvement is adding turn lanes or even through lanes.

Be leery of our last demand-side mitigation – decreasing the development. You’ve been hired to tell the city and developer how to make the proposed development work. The developer won’t be too excited about you changing his development plan, so this should generally be considered last, if at all. The agency may also want growth and more development, making them more resistant to a smaller size.

Test (and re-test) the Mitigation

Once you’ve settled on the initial mitigation, it’s time to see whether you have indeed solved the issues and have acceptable traffic operations in your capacity analysis. If your mitigation results in changes to the trip generation or distribution, start there. Adjust your forecasts and determine new volumes for the changed scenarios. Changed forecasts could result from new access points, restricted movements or a change to your development, to name a few. Think carefully about your mitigation and decide whether or not it will affect your volumes before moving on.

Then you can jump back into your evaluation software. Starting with the earliest scenario where you need mitigation, make the necessary roadway changes – and volume changes if you have them – and then rerun the analysis. Continue working your way through reevaluation of each scenario with your proposed mitigation. Once you’ve completed that, you can go back through the results and see what happened. Hopefully, you’ve solved your problem.

If your mitigation doesn’t solve the problem, the process starts over. Keep making lists of where your problems are and working on possible solutions. Eventually, you’ll have recommended improvements that allow the transportation network to operate acceptably in all scenarios.

Be sure to document as you evaluate and re-evaluate your mitigation. Write down each mitigation strategy and include to which scenarios each mitigation was applied. These will be important details to include in the study report. Also, keep your software files organized for each mitigation scenario. You can end up with dozens of options and get yourself into a huge mess if your files aren’t well organized.

If you decide a traffic signal, all-way stop sign, or roundabout installation is necessary, consider performing a warrants analysis per the Manual on Uniform Traffic Control Devices. Some jurisdictions, such as the Wisconsin DOT, require a separate Intersection Control Evaluation (ICE) report be prepared before they’ll allow a traffic signal to be installed on one of their roadways. An ICE report is a standalone document that analyzes all of the possible traffic control/geometric alternatives, historical crash data, and costs in order to develop a recommended solution.

Review Mitigation

If you’ve gone through the process of determining mitigation and evaluating multiple times, it’s often helpful to do a high-level overview of your choices afterward. Examine the mitigation for the full build-out scenario as well as what mitigation is needed for each interim scenario. Ideally, your sets of mitigation should naturally build on previous changes. For instance, if you add a turn lane in one scenario followed by a change to building a roundabout in the next scenario, would it make more sense to just go to the roundabout first without adding a turn lane? Don’t throw away your client’s money on very short-term fixes.

Again, this is an item that gets easier with experience. Just as you may have started out by talking with your supervisor, this is a good time for a final check-in before going to the client.

Meet with Client and Reviewing agency

You should set up a meeting with your client to discuss the mitigation. It’s always good to inform your client of the changes expected (which helps provide a magnitude of costs they may be expected to pay). Be prepared to defend your choices for mitigation. If your mitigation has been thought-out and can be properly defended through your analyses, you should have no major problems. In preparation for this meeting, it’s helpful to think about why you didn’t select some mitigation options over others. Being able to state why an option is not appropriate can often help you defend your choice of a different option.

If your client is not the governing agency, you may want to meet with city, county, and state staff as well. They may have insights into surrounding characteristics that may have an impact on the feasibility of your mitigation. As before, be prepared to defend your analyses and mitigation choices. Don’t be afraid to collaborate on particularly difficult situations. The City Engineer and other staff are great resources and will likely have opinions on potential traffic improvements. Generally, these people will know the area better than you and may already have ideas about future improvements.

This is a multipart series. Check out the other articles in this series.

• Traffic Impact Study Process – Part 1: Assumptions to Reviewers
• Traffic Impact Study Process – Part 2: Preliminary Assessment
• Traffic Impact Study Process – Part 3: Document Existing Conditions
• Traffic Impact Study Process – Part 4: Traffic Data Collection
• Traffic Impact Study Process – Part 5: Prepare Traffic Forecasts
• Traffic Impact Study Process – Part 6: Perform Capacity Analysis