Mike on Traffic

Traffic Forecasting - Past Performance Does Not Guarantee Future Results

I'm working on a traffic study where the reviewing agencies asked us to prepare 20 year forecasts in addition to looking at the build out year.  Typically, we look at data trendlines on nearby roads and throughout the county to determine a "typical growth rate" for traffic in the area.  This has historically been an annual growth rate of 1 to 3%.  This often leads us to factoring traffic up 50% or more over 20 years and then layering on the traffic from the proposed development.  Factoring existing traffic volumes up approximately 50% is also how traffic forecasts are often prepared for road design projects.

I'm strongly reconsidering this approach.  Consider Figure 1 below from the Federal Highway Administration's Office of Highway Policy Information website.  From 1986 to 2006, traffic on all of our highways did fit the model of growing by about 2 to 3% a year (or 50% to 60% over the 20 years).  But since 2005, we've had a significant drop in traffic and the trendline sure makes it look like traffic growth has plateaued. 

I've also been reading through the Texas Transportation Institute's 2011 Urban Mobility Report.  They talk about the terrible state of congestion.  I'm not going to argue that congestion doesn't suck, but take a look at Exhibit 1 from their report.  Compared to 1982, congestion is indeed terrible now compared to 30 years ago.  But looking at the 2000 to 2010 data shows congestion peaked in 2005 and we're not much worse off than we were a decade ago. 

Also consider the August 2011 to August 2010 data below from the Federal Highway Administration.  From their website, "Travel on all roads and streets changed by -1.7% (-4.6 Billion vehicle miles) for August 2011 compared with August 2010."   

But maybe Minnesota's different...... NOPE.  Below is Figure 1 from Mn/DOT's Vehicle Miles of Travel Trends in Minnesota:  1992-2010 showing traffic in Minnesota has followed the national trend in plateauing.

Based on national and local trends, my conclusion is that it is very reasonable to think traffic growth has plateaued.  The punchline for traffic impact studies: the "no-build" traffic forecasts should be the same as the existing traffic volumes.  We don't need to do opening day forecasts and 20 year forecasts because they can reasonably be expected to be similar. 

And given our huge budget shortfalls, this should also mean a policy of fixing the infrastructure we have.  NOT expanding our transportation system to add capacity.

Expert Witness Do's and Dont's

Tort law (at least in Minnesota) is very clear that road authorities have the right to place medians on roads to limit access to cross streets or driveways.  I routinely get called by lawyers and property owners trying to fight this type of project.  Nine times out of ten I tell them there's nothing I can do to help them (in the majority of these cases the median closure makes sense from a traffic engineering perspective and I assume the road authority has done their due diligence).

A few months ago I was hired in one of these cases, but it had a twist so it was the 1 out of 10 I work on.  The median had already been put in and the gas station who's driveway was closed is suffering.  The lawyer thought he might have found a loophole and they were in arbitration with the county over the project.

All I did was count the cars coming in/out of the site over 24 hours (thanks to our COUNTcam video systems) and then compare that to what we would expect for traffic generation at the site based on the ITE Trip Generation Report.  I was just benchmarking if the site was underperforming from a traffic perspective.  Turns out the site is only generating about 50% of the traffic that would be expected from a typical gas station.

This was a technically straightforward, factual study.  My client then had me present my study at the arbitration hearing.  I routinely present to city councils and county boards, but this was my first time as an expert witness in a "court hearing."  It went fine, but I found being cross-examined was more stressful than testifying at a public hearing (maybe it's just an experience level).

Now that I've been an expert witness, here's my top 10 do's/dont's for testifying (they apply to presenting to city councils too):

1. DO give your credentials.
2. DO tell the truth.  Stick to the facts.
3. DON'T speculate.  Answer "I don't know" when you don't know.
4. DON'T volunteer information.
5. DO stop talking if interrupted.
6. DON'T use jargon.
7. DO take your time to think.  Make sure the lawyer is done with their question.
8. DO answer with a yes or no when asked a yes or no question. 
9. DON'T answer questions you don't understand.  Ask for clarification.
10. DO cite sources when describing technical analysis.

 

How to Prepare Traffic Forecasts for Traffic Impact Studies

I attended the NCITE (North Central Section of ITE) planning methods committee meeting because traffic forecasting in traffic impact studies was on the agenda.  There was a good discussion, but not a lot of conclusions.  Below is a checklist that details out how we prepare our traffic forecasts.

• No-Build, Future Year Forecasts (Determine method for factoring up existing traffic volumes)
  • Typically use Mn/DOT’s assigned State Aid growth factor that’s been assigned for the County
  • Growth rate based on past traffic data in area
  • Growth rate backed out from transportation plan forecasts
  • Based on other area traffic studies
  • Combination of the above
• Volumes Due to the Development
  • Trip Generation (start with ITE Trip Generation)
    • Use average rates
    • Use best variable (i.e. employees or square feet?)
    • Make sure you’re using the right time frame (i.e. THE peak hour vs. the peak hour of the adjacent street traffic between 7-9 a.m.)
  • Trip Generation Discounts
    • Internal trips at commercial or mixed use developments
    • Pass-by/diverted trips – make sure they aren’t a large portion of the road’s existing traffic (rule of thumb – limit to 10% of existing traffic)
    • If there is heavy transit use or in a dense urban neighborhood, should you reduce the trip generation to account for alternate modes of transportation?
    • If you’re implementing a Travel Demand Management Plan for the development, should you discount the trip generation?
  • Trip Distribution
    • Make sure you distribute the traffic per your agreed upon trip distribution pattern.
    • Make sure the numbers coming in and out of the development site match the trip generation table you prepared.
• Build, Future Year Forecasts:  make sure the No-Build + Volumes Due to Development = Build Volumes
• Round your forecasts to the appropriate level of accuracy (round up)
  • Nearest 5 vehicles on low volume, peak hour movements
  • Nearest 10 vehicles on all other peak hour movements
  • Nearest 100 vehicles on daily volume forecasts
• Make sure the volumes going between adjacent intersections add up on all Volume Due to Development forecasts and between adjacent intersections in the No-Build and Build forecasts if there are no intersections between them.  Cars don’t levitate.

Vikings Site in Arden Hills - Initial Traffic Assessment

The Minnesota Vikings professional football (American football) team has honed in on a site in Arden Hills, MN.  You can read about the agreement with Ramsey County in the Star Tribune here (you can also hunt around the newspaper and other news sources for opinion pieces).  I'm very curious to see the traffic study that gets done for this (Mr. Wilf - we're available if you'd like our help with the traffic study). 

Below is a map that shows the location.  At first blush, the site appears to have very good freeway access from Interstates 35W and 694.  But if you look a little closer there are significant limitations for anyone trying to get to the site from the south or west (i.e. the majority of the fans).  Everyone from those directions will be forced to use the I-35W/Co Rd 96 interchange to get to the game. 

I hope they can get an off ramp built at Co Rd H, but that interchange doesn't have that move now because of the difficult weaving areas that are caused by Highway 10 and the close proximity to Highway 96.  I haven't looked into it enough to know if that's even feasible, but I assume Mn/DOT has.

Below is a scan of the conceptual site plan from my morning paper (I haven't been able to find this anywhere online - guess that's why I still get an old fashioned hard copy).  According to the team's website, the site will have about 21,000 parking stalls. 

One inbound lane can bring in about 2,000 vehicles per hour if the lane is unimpeded and traffic is rolling at a good clip (so we're being generous with this number, but it's a round number to use).  So, if we're trying to get the 21,000 fans into the site in a very chaotic 2 hours before kickoff, we better have 6 lanes of traffic rolling into the site.  The plan below is very conceptual, but I don't see that kind of access being provided. 

I hope the Vikings get a traffic engineer involved pretty soon with the site planning.  Unlike the current Metrodome site, there's no Light Rail Transit line to feed in the fans or the existing parking ramps available throughout downtown Minneapolis.   Since the site is a blank slate, the Vikings need to create all of the infrastructure to make this work.  At least there isn't much conflicting traffic for fans to contend with on a Sunday morning.

 

 

HCM 2010 Update - Stop Sign & Roundabout Webinar

Here are the slides from yesterday's webinar (Download Unsignalized HCM 2010 Presentation).  Highlights from the discussion -

Roundabouts

• This whole chapter is new.  We can finally compute Level of Service for roundabouts!
• Capacities at roundabouts in the U.S. are currently much lower than capacities observed in other countries.
• Calculations are based on simple models.  Experience in the U.S. shows roundabout performance is largely impacted by the variability in driver behavior, not specific geometrics configurations.
• Not included in the Level of Service calculations - geometric configurations (other than number/type of lanes), oversaturated conditions, or interaction effects of upstream/downstream intersections.  Other models should be used if these items are of concern.
• Delay thresholds are the same as those for stop sign controlled intersections, which are lower than the thresholds for traffic signals.
• On multi-lane approaches - the volumes in the two lanes are not assumed to be the same.
• There are calibration tools, which can ideally be tweaked based on local driving behavior data (this is true of the entire HCM 2010).

All Way Stop Sign Control

• Added methodology to model three lanes of approach instead of just two.
• New queuing model that is similar to the side street stop sign control method.

Side Street Stop Sign Control

• Added methodology to model six lane major roads instead of just two and four lane roads.
• Added methodology for modeling u-turns (our traffic counters will have more to keep track of).
• More refined method for dealing with the impact from upstream/downstream intersections.
• Better modeling of impacts of short left turn lane storage and large radii for right turn movements.

 

Checklist of Traffic Study Assumptions you should send to Reviewing Agencies

When time permits (assuming no one week deadlines), I find the most efficient way to work through the traffic study approval process is to start by sending the reviewing agencies the scope and assumptions that will serve as the foundation of your traffic impact study.

The review process usually (but not always if you have a flip-flopper at the agency) goes faster if you agree on the foundational aspects of the traffic study at the beginning stage versus revising your study later based on the review letter you'll get back from the agency.  This allows the reviewing agency to focus their review letter on the conclusions and recommendations instead of the nuts and bolts of the forecasts and capacity analyses.

If the agency will have a traffic engineering consultant review your study, make sure they're included in the discussion about the scope/assumptions.  You want to get confirmation from the actual people who will be commenting on the study, not their bosses boss.

 
I like to start with an email.  Often a few emails are enough to agree on the details.  But I've run into a few agency folks who prefer to iron out the details in a meeting.  As a starting point, here are topics I recommend you cover in your email:

1. Give an overview of the proposed development
   • Brief description of the development
   • A location map
   • A concept plan (highlight the proposed access points)
   • Anticipated build-out year
   • Phasing plan if the development will be built in stages
   • Detail the size of the development (i.e. square footage, dwelling units, etc.)
2. Provide the preliminary framework of your study
   • A map with the intersections and corridors you propose to study.
   • A map showing the distribution pattern the site’s traffic will use.
   • A table giving the traffic generation of the proposed development.  Mark it preliminary.
   • List the study periods you plan to study (daily, a.m. peak hour, p.m. peak hour, and/or Saturday midday peak hour).
   • List your proposed analysis years.
   • Document how you will increase the existing traffic counts to your future no-build scenario traffic counts.
   • List the methodology/software you’ll use for your analyses.
3. Request traffic signal timing plans if they have control over the traffic signals at any of your study intersections. 
4. Ask about (or confirm) proposed development and roadway projects planned within the study area.
5. Ask if they have policies, ordinances (especially traffic impact fees), and/or standards related to traffic studies.  If you found references online, confirm you have the current information. 
6. Provide your schedule and ask about their typical review process and its associated timeline (i.e. how much time will they need to review your traffic study).

Highway Capacity Manual 2010: Review of ITE Overview Webinar

I watched the Institute of Transportation Engineer's webinar yesterday that covered an overview of the new HCM 2010.  This is a huge change from the 2000 Highway Capacity Manual (explaining why the 2010 HCM is coming out in April 2011.....).  Here are the slides from the webinar (Download HCMoverview).

Here are some of the highlights from the webinar:

• HCM 2010 is supposed to ship this week.
• ITE will have ten webinars to go over individual components of HCM 2010 (more info here)
• The three hard copy volumes total 1200 pages
• New elements
  • Multi-modal approach (ways to analyze bikes, peds, transit in addition to vehicles)
  • Chapter describing when/how to use advanced traffic analysis tools such as micro-simulation
  • Refined Level of Service criteria based on current research
  • Primer for non-technical audiences
  • Analysis accounts for actuated controllers and left turn lane queueing at traffic signals
  • Tools for analyzing roundabouts (this is probably my favorite update - we finally have a simple apples to apples way of comparing stop signs/signals/roundabouts)
  • All way stop signs - include three approach lanes instead of just two
  • Side street stop signs - considers traffic from adjacent intersections
  • Quick estimation method for interchange ramp terminals
  • Volume 4 is an online site that contains discussion boards/errata/additions.  Tools to make this more of a living document.

I plan to attend about half of the upcoming HCM 2010 webinars (assuming NCITE continues to host them for a $10 fee).  I'll post a summary for each webinar I've attended.  I'll also post some thoughts on the HCM once I have a chance to go through my copy.  I'm also considering posting my login/password for Volume 4 so all of my loyal readers can log onto the website (without having to buy the $275 book).  Stay tuned...

Trip/Parking Generation Rates for an Urban Grocery Store

I'm preparing a traffic study for the proposed 25,700 square foot Lunds grocery/liquor/pharmacy store at Hennepin/12th in downtown Minneapolis.  We would typically use the Institute of Transportation Engineers' reports (industry standard data sets) to help us forecast the amount of parking needed at the site as well as the amount of traffic that will be generated. 

Based on discussions with the city, we think the standard data sets may overestimate the traffic/parking needs of the downtown grocery store since the existing data sets are largely based on suburban locations.  Lunds has another urban grocery store on the edge of downtown Minneapolis on Central Avenue.  Here is the data table from a p.m. peak hour study of that existing urban grocery store (Download Parking-Trip Generation at Central Lunds).

We found slightly lower peak hour trip generation rates and significantly lower peak period parking rates compared to the Institute of Transportation Engineers' reports.  This makes sense at an urban grocery store.  They still have a good volume of customers, but those customers are making a quick stop in for a few items and then getting out as fast as possible.  At more suburban grocery store locations, we'd expect patrons to buy more and stay longer.  Suburban grocery stores aren't as well set up for convenience stops.

Top 9 Things to Review with a Field Visit

Last week I posted about all of the great information you should get from online aerials.  So why do you need to go out in the field when everything is right there on the aerial?  First, because the aerials aren’t always right. 

I worked on a church project in a fast growing suburb.  We got out in the field and realized one of our study intersections didn’t exist and there were two new intersections we had to account for in our study.  The roads in the study area were rebuilt during the summer and the aerials weren’t up to date.  We had to adjust our data collection to account for reality.  Imagine if we turned in a whole traffic study based on the obsolete road network?  Our study would have had zero credibility.

Field Review of Physical Features

The second reason to go out to the site is that there are a lot of important details you can’t always see on an aerial.  In addition to verifying the information on your hand sketches is correct, add the following details to them while you are in the field:

1. Transit Stops
2. Traffic Signal Operations (Protected Left Turn Phasing is shown on the left, Protected/ Permitted Left Turn Phasing).
3. No Turn on Red Restrictions
4. Parking Restrictions
5. Speed Limits
6. Any road construction in the area that would affect your “normal” traffic counts?
7. Any construction or signs for developments near your site?
8. Excessive grades or slopes that make widening a road/ intersection difficult?
9. Any objects on corners of intersections that block a clear view of oncoming traffic or pedestrians?

While you are at the site, take plenty of pictures.  Get one photo of each approach to each study intersection (my iPhone has become a very useful all in one field tool with it's maps, camera, stopwatch, etc).  Make sure you keep a log of the photos so you can identify them back in the office.  Taking pictures has saved me more than one field visit.  It’s amazing how often one little detail is usually missed while you’re doing your field check. 

An intriguing alternative to taking pictures is taking your own video log.  I haven’t purchased a video system yet, but I am considering the Dual Car Camera Voyager Pro w/ GPS Tracking Logger.  It’s a camera, logger you mount on your windshield.  You download the recorded file from a removable SD Memory Card.  The software interface shows your video next to a Google^TM map that is tracking the corresponding car location.

Field Review of Traffic Operations

In addition to documenting the existing conditions of the environment, you should observe how traffic operates in your study area during rush hour.  I like to go through my hand sketches after lunch and then drive around the road network during the evening rush hour.  Things I’m looking for:

1. Do shoulders get used by cars as separate right turn lanes?
2. Do the traffic signals along a corridor seem to be coordinated so through traffic moves down the corridor smoothly?
3. I take a stop watch along and do a few spot checks of each traffic signal cycle.
4. Are there any queues that back up out of turn lanes into the through lane or do queues extend back from one intersection blocking the upstream intersection?
5. Is it hard to turn onto a major street from a cross street that has a stop sign?
6. Think about the routes cars will take to get to/leave your site.

These firsthand observations will save you in a public hearing.  I’ve been put on the spot about citizen’s observations of how bad traffic is.  If I’ve been out there during rush hour, I can come back with “I went out during rush hour and I saw that, here’s how I incorporated that into my recommendations…. (or I didn’t see that, how often does that happen?).” 

And of course, the field visit is part of your quality control process to ensure the details in your traffic study are correct.  The quality control process in my office is:  (1) the technician draws the sketch based on the aerials, (2) she verifies/corrects the information during a field visit (she also figures out if we need to change the data collection plan as well as determines good parking spots for our data collectors), and (3) I go back to verify her drawings (usually she has created the existing conditions diagram we’ll use in our traffic study report by this point) while I am out to observe rush hour traffic operations.

11 Items to Get From Aerials When Preparing for a Field Visit

Once you’ve been given a traffic study to work on, you have two tasks to get done in short order – (1) order the traffic counts you’ll need and (2) visit the site to document the existing conditions.  You may need to tweak your data collection plan based on your field visit, but I recommend you get the data collection on the schedule ASAP and adjust as needed.  That parallel process has allowed me to get traffic studies done faster than doing the linear field visit then traffic count process. 

To do your field check as efficiently as possible, I recommend you start with your favorite aerial online site or software (Mapquest^TM, Google Earth^TM, Bing^TM, Google Street View, County/City/State GIS Maps, etc.).  It's actually best to look at a few online aerials to find the most current one.  I especially like the angled birds eye view Bing offers and Google's street view. 

Draw a sketch of each study intersection based on the aerial (a quick and dirty pencil sketch by hand will work) with the following details:

1. Exclusive left or right turn lanes (measure their storage lengths and include it on the sketch)
2. Through lanes (and/or shared lanes if turn lanes aren’t provided)
3. Stop sign locations
4. Traffic signals
5. Medians
6. Shoulders
7. Sidewalks/trails/paths/crosswalks
8. On-street bike lanes
9. Nearby parking lots
10. On-street parking
11. Street names

If time permits, I have my technician create a figure from my hand sketch that I can take in the field with me (such as the one below).  Also print out a street map of the study area to take notes on.  Make sure it is zoomed in enough so you can make notes of things that are between your study intersections. 

I find I am more accurate correcting my sketches in the field than trying to draw them from scratch while I am on the side of the road.  I always seem to miss something if I skip this first step in the office. 

Next week I’ll post a checklist of what I look for when I'm out at the site.