Mike on Traffic

PC-Warrants - Forecasting Hourly Volumes for Signal Warrant Analysis

Back in March I wrote about forecasting hourly volumes for signal warrants from forecasted daily traffic volumes.  It's come to my attention that PC-Warrants has a tool built in it that makes the process very easy.  If you have PC-Warrants, search under "Volume Data Wizard - ADT to Volume Converter." 

The Volume Wizard lets you put in your own hourly percentages to convert the daily traffic volume to the hourly volumes.  The default percentages in PC-Warrants are  "Default values for typical urban approach, from Pignataro's 1973 textbook."  Here's a screenshot for the data entry window.

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.

Numbers Every Traffic Engineer Should Know

I'm in a lot of meetings where I'm seen as the expert on traffic issues (shocking, I know) and I get asked  questions related to basic standards and general practice.  You can always respond you don't know the answer and you need to look it up, but you look better if you're able to rattle off the numbers from memory.  To that end, here's a list of questions and answers every traffic engineer should consider memorizing -

About how much traffic will my development generate? (round numbers based on ITE Trip Generation Report, 8th Edition)

• Single Family Houses (per unit):  10 trips per day, 1 per peak hour
• Apartments/Condos/Townhouses (per unit):  6 trips per day, 0.6 per peak hour
• Office (per 1000 sq ft):  11 trips per day, 1.5 per peak hour
• Retail (per 1000 sq ft):  43 trips per day, 3.7 per peak hour
• Industrial (per 1000 sq ft):  7 trips per day, 1 per peak hour

What's the daily capacity of a road?  (Round numbers based on Level of Service D/E thresholds in HCM 2010)

• 2 lane local street:  1,000 vehicles per day based on livability
• 2 lane (w/ left turn lanes):  16,000 vehicles per day
• 4 lane (w/ left turn lanes):  31,000 vehicles per day
• 6 lane (w/ left turn lanes):  44,000 vehicles per day
• 4 lane freeway:  72,000 vehicles per day
• 6 lane freeway:  108,000 vehicles per day
• 8 lane freeway:  145,000 vehicles per day

What's the peak hour capacity of an intersection?  (Based on Level of Service D/E thresholds in HCM 2010)

• Stop sign controlled:  35 seconds/vehicle
• Roundabout controlled: 35 seconds/vehicle
• Traffic Signal controlled:  55 seconds/vehicle

Theoretical maximum saturation flow rate per lane? (this will allow you to do quick calculations in your head to check reasonableness at big events) - 1900 vehicles per hour per lane 

What's the threshold for when you need to add a second (dual) left turn lane at a signalized intersection?  300 left turning vehicles from that leg of the intersection in the peak hour

How wide should a commercial driveway be (based on NCHRP Report 659)?

• One lane in only: 14 feet curb to curb
• Two lane, bi-directional:  24 feet curb to curb
• Three lane, one lane in with median than two lanes out: 40 feet curb to curb
• Minimum industrial driveway:  26 feet curb to curb

How big of a footprint does a roundabout take?

• Single lane diameter: 132 foot radius
• Double lane diameter:  165 foot radius

How close can we have driveways/intersections? (these are very rough rules of thumb since this is very situational)

• On a local street:  150 feet
• On a collector street:  660 feet
• On an arterial:  1,320 feet to 2,640 feet (with medians, right-in/right-outs can be 660 feet away)

 

 

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.

 

Lessons Learned - Experience from Minnesota's first Multi-Lane Roundabout

I saw Joe Gustafson, PE, PTOE of Washington County give a great presentation at last week's NCITE meeting.  He talked about the County's experience with Minnesota's first two lane roundabout (at least the first roundabout with two lanes on all approaches). 

Joe shared his powerpoint (Download Roundabout 2-Year Study NCITE Oct2011) with me and also pointed me to an online edition of the presentation (this was a rehash of a presentation he originally gave at TRB).  If you work with roundabouts, I highly recommend you watch the 20 minute online edition of the presentation.  Per minute, it's the most informative roundabout training I've had (and I've sat through more than 60 hours of seminars and classes on roundabouts in the last 11 years).

A few highlights -

• Compared to a nearby signalized intersection that is very comparable in traffic volumes and entering roads, the roundabout adds about 12.5 seconds of control delay to the average through movements in the p.m. peak hour while the signal adds 29.5 seconds of delay to comparable through movements.  The left turns operate even better - 18.1 seconds of control delay at the roundabout versus 43.7 seconds at the signal.  The roundabout is operating a lot better during rush hour than the signal.
• No injury crashes, no wrong way crashes, sideswipe crashes are rare, crashes where cars run into the center island are rare.  However there have been 140 fender benders in the four years since it opened.  City of Woodbury police is keeping a separate, detailed record because most of the crashes don't even get reported to the Department of Public Safety. 
• Fender bender rate is high.  There is a systematic failure of people coming into the roundabout to yield to the people in the inside lane who are legally exiting the roundabout (see picture).  Joe has a great explanation of the crashes in the online edition of his presentation.

Lessons Learned -

• Our industry needs to be consistent with our message - entry and exit maneuvers are through movements at a roundabout (not merges or turns).
• Don't build more lanes than needed.
• Don't refer to the "circle" as one road.  Think of it as four different intersections, each one made up of two crossing one-way streets (see diagram).
• Roundabout signs - use standard turn arrow signs, but tweak by adding a dot representing the roundabout (don't try to use "fish-hook" and other confusing signs).
• Use 8 inch stripes at exit lanes for better visibility.
• Do a lot more human factors testing on methods for conveying that the entry into a roundabout is a straight movement (not a merge or right turn).

Joe - thanks for the great presentation and for sharing your materials with me!

Real World Sign Retroreflectivity Tests

Thanks to federal regulation, all agencies need to keep an accurate sign inventory and ensure their signs meet certain retroreflectivity requirements (translation - know what signs you have and make sure the signs reflect enough light at night so motorists can see them). 

Robert Wunderlich (City of Garland Texas Senior Managing Director of Transportation/Engineering along with being ITE's International President) posted the following test results on ITE's Traffic Engineering Council bulletin board on the ITE Community page (if you're a member of ITE you really should get plugged into the ITE Community page - it's a great resource).  Thanks Robert for letting me share this!

Dear Colleagues:

Thanks to our friends at the City of Denton, Texas, we had the use of a retroreflectometer (RRmeter) for the past two weeks. 

We used it to see if we could use some leftover Engineer Grade (EG) materials to make comparison panels or calibration signs and I did a little nighttime measuring in my neighborhood.

New Engineer Grade Test Results

Color	Min	EG measurement
White	120, 50, 35	123-125
Yellow	75	76-78
Red	7	17-18
Green	15	15

So, we have a lot of what we need already. Most of the paper exceeded these values.  We had to dig for some of this. We're looking for some older white signs that might be closer to the 50 and 35 values.  Most of the new EG white was around 100 cd/lx/m2.  It's hard to find new EG red lower than 17 or 18, but stopping's pretty important so setting a threshold around there is probably OK.  Most green is above 15 but we found a portion of a roll that measures 15. 

Rolls tend to vary considerably and it absolutely impossible to determine what the RR value will be just looking at it during the day.

Field Measurements

I highly recommend taking a RRmeter out during the night and checking signs.  It brings these values to reality and there's no substitute for going out and understanding what they mean.

Mr. Robert's Neighborhood (not in Garland, by the way)

I found New Hi-Intensity sheeting exceeds the minimums by an order of magnitude.  Signs really have to be quite bad to not meet the minimums, although the 3:1 white:red ratio gets some otherwise acceptable signs.  I will say that these signs do not reflect a bright, deep fire engine red (like a new Hi-Intensity one does) but it's not going to get confused with any other color either.

Quite frankly, I believe that any sign that doesn't meet these minimums would be judged unacceptable by any of us or our sign crews.  In fact, our crews probably would have much higher thresholds than the minimum.  Lot's of old EG signs still meet the minimum (even when they've been egged, and boy, there's a lot of that out there)  There's no way to tell what a sign will reflect at night by looking at it during the day.  Some signs that look pretty crappy in the day meet the RR mins.  Not saying that we should keep crappy looking signs up.  Also, I will say that even for the stop signs that didn't meet the minimums, it would be hard to say that you didn't know what you are supposed to do there (i.e., stop)  It's red, its octagonal, and it's not invisible (I mean really).

9-year old Hi Intensity Ground Mounted Street Name Signs (in Garland)

10 years ago, we started a comprehensive replacement of our really crappy (there's that technical term again) street name signs.  We finally got some funding when the Public Safety folks said they couldn't find the fires and the crooks cause they couldn't see the signs.  (See, we are part of Public Safety!)

Today, we checked a few of the signs in an area replaced in the second year.  These are hi-intensity beaded sheeting signs.  Our green RR values ranged from 43 to 47 (with no correlation to whether the sign was south-facing)  The ground-mounted green minimum is 15 and a 10 year-old sign made from the same material that has been up in our sign shop measured 66. 

The white values ranged between 210 and 236 (minimum 120).  The like-new sign in the shop measured 304.

So, we've had some deterioration but we're still well above the minimums.  The warranty is almost over and the signs will continue to function adequately.  The durability testing results I've seen indicate that the deterioration does steepen, but I'm betting we get 10 more years out of these signs.  We'll see (so to speak)

We did this project over about 5 years, and we plan to get more data to see what the deterioration has been.

Lite Research Summary (what I think I know now)

RRmeters are pretty cool.  Gotta get me one.

We are in pretty good shape with our street name signs.  Will they last till 2018?  I'm thinking they will, but eventually we will need to replace them, and we've got a ton of them.  Not an emergency.

Warranty does NOT equal sign life.

I'm not going to blanket replace.  I can't see replacing functioning signs.

If the white is good on a stop sign, it's pretty likely the red will be ok.

I'm concerned about the yellow warning signs.  I'm also concerned that most of them weren't needed in the first place and now we gotta replace 'em.  (fer crying out loud, maybe we should just take them down)

More when I have it. 

That's all for now.  Gotta go watch the Rangers win Game 3 of the ALCS.

Robert

P.S.  Please check out Paul Carlson's excellent RR FAQ's.  Paul is a leader in this field and has a very practical take on this.  He's been a great help to me in understanding what is and what ain't the requirements.

-------------------------------------------
Robert Wunderlich P.E.
Sr. Managing Dir.- Transportation/Engineering
City of Garland
Garland TX
robert4ite@gmail.com

Self Assessment of Traffic Signals for National Traffic Signal Report Card

Improving signal timing has about the best benefit/cost ratio of any improvement the traffic engineering community can make.  Back in 2005, a bunch of agencies collaborated to put together the National Traffic Signal Report Card.  The second edition was released in 2007 and now the group is polling all agencies involved with traffic signal timing to assemble the third edition. 

According to the report card, the United States went from a D- in 2005 to a D in 2007.  The 2007 report highlighted case studies of how agencies could make small (cheap) changes to raise their grades several levels.  Traffic engineers need to use reports like these to educate their policy makers on the huge traffic management benefits of having well maintained traffic signal operations so budgets can support these efforts.

If you are involved with traffic signal timing, I hope you'll spend the time to fill out the assessment here. 

Intersection Spacing - Roundabout/Dual Left Lane Integration

Landform is working on an updated master plan for The Cor (a large mixed use/neo-traditional development in Ramsey, MN that has struggled due to the economic downturn).  They brought me in to update some traffic forecasts which Mark Lenters (Ourston Roundabout Engineering) can use to redesign a key intersection in the development. 

How close can an intersection controlled with a roundabout be to another intersection?  Based on operational experience around the country, much closer than if the intersection was controlled with a traffic signal - often a couple hundred feet is all the spacing that's needed.  This is pretty well established and why Mark was brought in for this project, but how close can a roundabout controlled intersection be to a traffic signal controlled intersection that has dual left turn lanes at it?

I raised this question to Mark early in the project and he's been kind enough to share the real world example (below) at Utah Valley University in Orem, Utah.  There's only 200 feet between the roundabout and University Parkway.  I haven't found any references to how well this area operates, but it's a creative example of putting a roundabout close to a large intersection.

How to Professionally Review a Traffic Study

I've reviewed several traffic studies for communities and I've had a lot of my traffic studies reviewed by other engineers.  Most of the reviews I've seen are a laundry list of 20 to 30 bullet points that don't always give clear guidance to the city.  I think the outline below provides a very clear and actionable memo format for the city staff who hired the independent consultant.

Section 1 - Introduction/Purpose

Give  a brief description of the traffic study and the purpose of your memo.  Say who prepared the traffic study and whether or not they are qualified to prepare traffic studies (are they a Professional Engineer, Professional Traffic Operations Engineer, etc.).  Keep this section to three or four sentences.

Section 2 - Conclusions/Recommendations

Clearly say whether or not the traffic study is sufficient or if further study is needed.  If further study is needed, list the things that need to be covered and the express purpose of covering those items (describe what you're concerned about).  If the traffic analysis was done properly but you disagree with the conclusions/recommendations, clearly state why your disagree and what you recommend. 

Section 3 - Technical Review

This is where the bullet point list comes in. Go through each component of the traffic study and state whether or not you agree with the study.  Here is where you can flag typos and minor imbalances in traffic numbers.  If the city/county/state has written policies or standards related to traffic studies, they should be cited and inconsistencies should be identified.  When flagging an issue or disagreement, clearly identify whether it is a minor issue that wouldn't affect the conclusions/recommendations of the traffic study or if they are a major issue that warrants further analysis. 

Lastly, as part of doing a good review, visit the development site to make sure all of your comments are accurate.  It kills your credibility if your review has mistakes in it.

The Arms Race of Signs - Bigger Is Better!?!?

I've lived in my house in St. Louis Park, MN for almost nine years now and I drive through the unorthodox interchange at Highway 100 and Minnetonka Boulevard on a daily basis.  This year, MnDOT "upgraded" the signs on the southbound off ramp.  Below are photos of what they put in. 

This is in no way scientific, but no one I know (me, my wife Jane, or several neighbors I've asked) have ever seen a car parked on this off ramp or seen a car drive the wrong way. I'd like to learn more about the documented problem these signs are solving.

The signs are huge and frankly obnoxious in a neighborhood setting.  Maybe these signs do solve a  problem, but to my semi-educated eye they seem very out of place and a waste of money.  In these days of limited resources, traffic engineers should be trending towards minimilasts when it comes to signs.