Mike on Traffic

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.