Mike on Traffic

Free Exhibitor Guest Passes to NATMEC: Improving Traffic Data Collection, Analysis, and Use Conference

Once again proving how much the industry likes acronyms - The TRB's biennial NATMEC (Transportation Research Board's every other year North American Travel Monitoring Exposition and Conference) will be held in Dallas June 4th-7th.  

CountingCars.com will have a booth there and we have 25 guest passes to give away.  Email your business card to mspack@countingcars.com and I'll get you a free pass to the exhibits.  Of course, first come first serve on these hot tickets.

 

New Traffic Safety Analysis Manual

There are a lot of really good NCHRP reports on traffic safety, but they are pretty specific in addressing a specific aspect of safety.  The Institute for Transportation at Iowa State University has put together a great summary on traffic safety - the Traffic Safety Analysis Manual.

The target audience for the Manual are public agency engineers, not specifically traffic engineers.  The intent is to provide a basic primer with a lot of useful checklists and a step by step process for improving safety.  The authors hope city and county staff engineers will be able to use the Manual to improve safety in their jurisdiction.

The first few chapters are pretty Iowa specific, but the later chapters provide general information any engineer can use.  My favorite sections are the checklists of Issues/Countermeasures on pages 40-44 and 80-84.  This is a very handy reference.  Check it out.

 

Student Apartments - Traffic Generation

There's been a housing boom around the University of Minnesota for about three years.  Very nice, new apartment buildings are being built with underground parking garages.  The new trend seems to be that each student wants their own bedroom and most of these buildings are providing a range of unit sizes - a mix of studio to 4 bedrooms.

We've provided traffic studies/travel demand management plans on several of these proposed buildings.  One thing we wrestle with is what trip generation should we use?  We've used trip generation numbers from ITE's Trip Generation for the generic apartment land use, but that seems high.  Enough of these buildings are finally operating now that we decided to do a little trip generation study.

We were able to track down six buildings where we could easily video their parking lot driveways AND get building statistics.  We've determined a.m. peak hour, p.m. peak hour, and daily trip generation rates Vs. number of dwelling units, number of parking stalls provided, and number of bedrooms provided.  Read our full study here  Download Student Apartment Trip Generation Study.

Below are the quick results compared to the average rates of a generic apartment building in ITE's Trip Generation, 8th Edition.  It's clear student housing generates low levels of traffic and a full traffic impact study will only be justified for very large complexes (roughly more than 400 dwelling units).  We were not able to capture bicycle data, but in our interviews of property managers it seems bicycle parking is a much bigger transportation issue than vehicle parking/traffic impacts.  Something designers should keep in mind.

Average Trip Generation Rates for Student Housing and Apartment per Number of Dwelling Units

	Student Housing Apartments	Apartment from Trip Generation, 8th Edition
Weekday	2.82	6.65
Weekday A.M. Peak Hour(between 7-9 a.m.)	0.13	0.51
Weekday P.M. Peak Hour (between 4-6 p.m.)	0.24	0.62

 

 

When to Install Protected Left Turn Phasing at a Traffic Signal

Dale Robinson emailed me wondering why some traffic signals with little traffic have protected left turn phasing (solid red arrow) while other signals have protected/permitted or even permitted left turn phasing (just a green circle).  He thinks this is a waste of time and increases vehicle emissions.  He's right!

Standards have evolved over time and often this decision comes down to engineering judgment.  Some traffic engineers don't trust drivers and want all left turn lanes at signals to have protected left turn phasing.  I disagree with that approach and fall in line with NCHRP Report 457, which says on page 42 - "the number of phases should be kept to a minimum because each additional phase in the signal cycle reduces the time available to the other phases."

So traffic engineers should always start with trying to get away with permitted phasing (no arrows).  If that won't work I'm a big fan of the Flashing Yellow Arrow protected/permitted left turn phasing.  This gives motorists a permitted signal most of the time so they can use their judgment, but if traffic is so heavy they can't turn it will eventually give them a green arrow.

There are a few conditions when protected (red/yellow/green arrows) should be used.  NCHRP Report 457 has a good decision tree in Figure 3.5.  Basically, go with protected left turn phasing only if you have one of these conditions:

• The movement has exceeded the critical number of crashes with the protected/permitted left turn phasing, so protected phasing is required to solve a documented safety issue.
• There isn't enough sight distance provided - meaning the motorists can't see oncoming traffic for a long enough period to judge if they'll have enough time to make the turn.
• There are two or more left turn lanes on the approach or the opposing approach.
• The left turn opposes more than three through lanes.

These situations are pretty rare except at big suburban intersections.  Most left turn lanes at signals should have permitted phasing or flashing yellow arrow protected/permitted phasing.