Saturday, April 25, 2015

Signal Timing - the Phoenix Experience

One of the most difficult, challenging, hair-pulling, and rewarding experience is to completely re-time a city. Over the years, I have been blessed to be involved in many re-timing projects.  Timing a city provides a great benefit/cost ratio because for a small amount of money, you can gain huge user benefits.

When I say signal timing, I am not simply talking about settings in the controller - walk, don't walk, clearance, all-red, etc.  I am talking about creating cycle lengths and offsets for the safe and efficient movement of traffic along a corridor.  It's one of the most challenging projects a traffic engineer will ever undertake.

Normally, the process goes like this.  First, you collect traffic data along major corridors and at intersections.  Then, you will typically find logical progression corridors or grids.  The next step involves using signal timing software to optimize the timing.  Finally, these timings are loaded into the system.  Fine-tuning is necessary because as you change the system - you will likely modify the behavior of the motorists.  If motorists are suddenly faced with different timing, they might speed up or slow down - depending upon what they see.

I have seen some cities never perform major traffic signal timing projects.  For example, Phoenix is laid out in a grid pattern.  There are typically traffic signals every mile, half-mile and sometims quarter mile.  And it turns out that a 90 second cycle length provides incredible progression in all directions.  That is because it takes about 90 seconds to travel one mile at 40 miles per hour. If the capacity and phasing of the major intersections can be controlled with a 90 second cycle length - then this provides a perfect backbone for signal timing.  Simply time all of the intersections the same and put in a 90 second cycle length - you'll get pretty decent progression.

For years this worked in Phoenix - mostly because for a long time there were few left-turn phases.  But now that exclusive left turn phases are at almost every major intersection - the 90 second timing no longer works.  There isn't enough phase time to accommodate all the phases.  So even Phoenix now should perform signal timing on a frequent basis.

I am not sure how many cities perform signal timing overhauls.  As traffic patterns change and volumes increase, it can have a dramatic effect on the system.  In my opinion, these overhauls should be done every five years at least.  But budgets are tight, the work is difficult, and few people will recognize the difference.  It's not the flashiest project in the world. But the benefits are huge.

Bill Reilly Story

I'll never forget when I met Bill Reilly.    In the late 1980s, he was a big name in the development of the Highway Capacity Manual.  I think he was the chairman of the signalized intersection committee. This was when Dolf May was the overall chairman.  I was a newbie traffic engineer doing work out of Phoenix, Arizona for Lee Engineering.

Anyway, we had been measuring traffic flow rates at intersections in Phoenix.  We had determined that the 1800 Ideal Saturation Flow Rate (The rate at which vehicles move through an intersection) was too low.  But the Saturation Flow Rate was also modified by these factors that were listed in the Highway Capacity Manual.  Each factor lowered the flow rate by a bit.  One factor, I think it was the truck factor, really substantially lowered the flow rate.  But no matter how hard I searched, I couldn't find any research documenting this particular factor.

It turned out that one year, I was to present one of my papers to the Transportation Research Board.  I was excited to travel to Washington, D.C. to present the paper, but I was also excited to attend the Highway Capacity Meetings.  Because I would be able to find out where the research was to back up the factors.

Well, the meeting wasn't really the right place to bring up this issue, and so after the meeting, I boldly went up to Bill Reilly and asked him where they came up with the factor.  His answer surprised me, to say the least.

"We made it up", he said.  "We needed a factor and we didn't really have any research, and so this is what the committee guessed".  I was astonished.  It had huge implications across the United States and there was no data to back it up?  Amazing.

Of course I now understand that much of engineering is educated guesswork.  You can't have a procedure or factor for everything.  That's why experience and grey hair is really quite valuable.  There's nothing that will give you that experience except monitoring and studying traffic for a career.  Bill Reilly certainly did that.

We sent our research into the Signalized Committee and I think this prompted more research into the area.  But there is nothing wrong with the approach that the committee took.  They needed a value and they took their best guess.  It was certainly a better guess than I would have made.

Loop Detection

I remember in college my friend, Steve, lived in a dormitory on the other side of campus.  To get to his dorm room, we had to cross a major street.  There was a pedestrian crossing but it was not actuated.  It simply provided a pedestrian phase in synch with traffic progression along the street.  In other words, it provided a pedestrian crossing every 120 seconds or so.

But as engineering students, we didn't know about traffic engineering yet.  Someone had told my friend that the "traffic" pullbox was really a pressure sensor.  If you wanted to cross the intersection, you simply needed to jump up and down on the box and soon you would get a green phase.

It worked!

 If we jumped up and down on the pullbox, we would get a green phase... about every 2 minutes or so.  Remarkable.  The sad thing is that I believed this until I became a traffic engineer.  I look back and wonder how dumb I must have been.

There are many ways to actuate a signal now, but back then, almost everything was done by loop detection.  A loop detector is a wire coil embedded into the street.  A small current is constantly being passed through the loop creating an inductance field.   When a vehicle passes over the loop detector, it creates a small disturbance in the field which is amplified through a loop amplifier and the signal is then sent to the controller.

The loop detectors are quite noticable at most intersections - as it is usually placed into the intersection by sawcutting the pavement.  This leaves a terrific scar on the roadway.  People know this scar has to do with vehicle activation, but they aren't really sure as to how it works.

Over the years, I have heard some pretty amazing theories.  I have heard that there are magnets in the roadway.  I have heard that they detect weight.  I have heard that they detect temperature, sound, and exhaust.  But nope.  They simply detect the change in the field created by the current in the loop.

I bring this up because of a recent article about loop detectors and bicyclists in Chicago.  The article states "Motorcycles and bicycles often aren’t big enough to trigger magnetic sensors that switch traffic lights from red to green, WBBM Newsradio’s Alex Degman reports."

Actually, they aren't magnetic sensors in the roadway.  The inductance field is a magnetic field, but theren't aren't any magnets in the roadway at all.  But, if you jump up and down really hard...

Saturation Flow Rate

There is a fancy term to describe the maximum number of vehicles that can travel through an intersection.  It's called the "Saturation Flow Rate" or SFR.  In 1985, traffic engineers believed that unimpeded queued vehicles would travel through an intersection about once every 2.0 seconds.  Based upon that, and probably a few studies, the Saturation Flow Rate was set at 1800 vehicles per hour.  (This is obtained by dividing 3600 second/hour by 2.0 seconds/vehicle). It was a nice round number.

There was a problem with this value, however.  When we did traffic studies at very busy intersections, we found that traffic moved through the intersection faster than this value.  Sometimes our calculations left us scratching our head.  We would measure traffic through the intersection greater than the calculated capacity.  What was going on?

The answer is that the 1800 Saturation Flow Rate was too low.  Today it is set at 1900, but in 1985 the rate was 1800.

Now the 1985 Highway Capacity Manual method was correct.  Given all the inputs into a capacity calculation (lost time, cycle length, saturation flow rate), it will fairly accurately calculate average vehicle delay. But the 1800 SFR flow rate was not correct.  It was too low.

In the late 1980s, we performed research of saturation flow rates at major intersections in the Phoenix area.  What we found was very surprising.  At busy intersections, we found that vehicles traveled through the intersection at 1.8 seconds per vehicle.  And the results at some intersections were astonishing.

One major roadway through the Phoenix area is Grand Avenue.  Back in the 1980s this road created several six-legged intersections. Since each roadway needed its own phase, the cycle length at these intersections could be almost three minutes.  So when vehicles received a green light, they went.  And boy did they go!

We measured Saturation flow rates at Grand Avenue and found that they were consistenlty between 1.6 and 1.7 seconds per vehicle.  This translated into a Saturation Flow Rate of 2200 Vehicles per hour.  This was substantially greater than the 1800 Vehicles per hour that was imbedded in the 1985 Highway Capacity Manual.

Today, the 1900 might still be low.  Most agencies don't have the resources to measure Saturation Flow Rate, but they should.  It gives a more-accurate picture of the actual traffic situation.

Photo Radar Insanity

Pima County announced yesterday that it is considering adding up to 12 additional photo-radar sites. From the article, it appears that the criteria for adding the sites is this:  They will be inexpensive to install.  Boy does this get my blood boiling.

You see, Photo Radar can be expensive to install because it requires underground conduit.  But it is much cheaper to install when a roadway is being ripped up for another construction project.  From what I tell, this is why these locations are being chosen for Photo Radar – It will be inexpensive to install because of other construction along the roadway.

But I have two problems with this.  First, is the concept of photo-radar altogether.  As near as I can tell -they are almost always installed at speed traps.  What is a speed trap?  Let me explain.

A speed limit is a posted speed along a roadway that creates a prima-facie evidence that you are driving recklessly.  Before speed limits were introduced, a police officer had to prove that the motorist’s rate of speed was reckless.  This could be very difficult.  So to facilitate this, traffic engineers conducted speed studies along all the roadways within their jurisdiction.

The general principle is this:  Most people will drive at a safe rate of speed.  So the traffic engineer would determine the rate of speed at which 85% of drivers would not exceed. This was done by measuring the speed of every vehicle along the roadway.  Then, the engineer would round this up to the next highest 5 mph.  So if 85% of the people were driving 42 miles per hour, the speed limit would typically be set at 45 miles per hour.

This system really worked well.  If the speed limit was set at 45 miles per hour, and someone was driving 60, he was obviously being reckless and warranted a ticket.

The problem comes when a speed study is not done.  In many instances now, the speed is simply set by someone estimating what a proper speed should be.  Here in Vail, we have a road, Mary Ann Cleveland Way.  This is a two-lane road that goes through 4 miles of wilderness.  Now there are bike lanes along the side of the road (That deserves another post) and so the risk of an accident with a bicycle is real.  But the road is posted at 45 miles per hour.  When you drive the roadway, it is almost impossible to drive 45 miles per hour.  In fact, I have to set my cruise control to make sure I don’t speed.  If the County were to perform a speed study, I’m sure the speed limit would be set at 65 miles per hour.  So, when people drive what comes naturally to them, they are “speeding”.  If Pima County wants to give out a lot of tickets, Mary Ann Cleveland Way is a place to do it.  It is a speed trap.

As I have observed the location of permanent photo-radar locations, in every instance they have been placed along speed traps.  So they really serve no other purpose than raising money for the County.

But secondly,  if you are going to place photo-radar at all, they should be based on a better criteria – say accident rates. The County could review accident rates along all of the major roadways and find locations where accidents are high due to speed.  This does happen sometimes...  If there is a blind curve or a poor sight distance.  The County may not have money to fix these locations – so a photo-radar system may actually be of benefit.

But according to the news article, the location of the proposed photo-radar is not based on accident locations.  It isn’t based on anything other than the fact that it will be cheap to install.

That is not right.  Because rather than adding photo-radar to decrease accidents, they are being considered for economic gain.

HAWK Signal

The city of Tucson, AZ, developed the High intensity Activated crossWalK (HAWK) pedestrian crossing beacon in the late 1990s to assist in pedestrian crossings, especially for major arterials at minor street intersections.

When I first moved to Tucson, I had never seen this signal before.  In fact, based on my limited knowledge of the Manual on Uniform Traffic Control Devices (MUTCD), I thought the signal was likely illegal.  Traffic Signals are not allowed to remain "dark" unless they are bagged.

But my biggest concern was in what to do while this intersection is flashing red.  My interpretation of this signal indication is that the crosswalk should be treated as a stop sign control.  However, all of the motorists treat it as yield control - initially stopping but then cautiously entering the crosswalk without stopping after the pedestrian passes.  But over time, I respect the signal and my guess is that it is extremely effective.

Traffic Signal Engineers are faced with a dilemma when it comes to pedestrian crossings.

 According to the Model Traffic Code (adopted by most agencies), a pedestrian has the right-of-way at any intersection - whether it is striped with a crosswalk or not.  But pedestrian behavior is erratic.  When there is no crosswalk, the pedestrian takes extreme care when crossing a road.  But when there is a crosswalk, most pedestrians believe that the white stripe on the ground will protect them.  It won't.

So the HAWK signal is a great tool for the toolbox.  It creates a safe pedestrian crossing that is only activated when needed.

What most people don't see, however, are the trade-offs.  When the crossing is activated, it creates huge motorist delay.  This delay isn't just because the motorist must stop at the HAWK crossing, but also because they will then become "out-of-sync" with the queue.  If the road is timed for optimal progression - it could actually cause two additional stops.  It is the stop that really annoys motorists.  It also creates a huge roadway user cost.  If a benefit/cost analysis were done of this traffic signal control, I am sure it would not be positive.

The Federal Highway Administration has been in review of this concept for some time.  They just recently released a Safety Effectiveness of the HAWK Pedestrian Crossing Treatment. The report documents the experience in the Tucson area.  I plan to read it and give further thoughts.

Roundabouts in the US

I first became intrigued by roundabouts based upon the Colorado experience.  As a traffic engineer living in Denver, I was keenly watching as the Town of Avon decided to place five roundabouts in their community to relieve traffic congestion.  This was a risky move.  The interchange of Avon and Interstate 70 was horrible.  During the peak hours, traffic would back up onto the interstate - af very dangerous situation.

But the roundabouts were going to change all this - at least the traffic engineers hoped so.

The local newspaper was not convinced.  They ran editorial after editorial denouncing the proposed solution and claimed that any lives lost would be the fault of these roundabouts.  I was impressed by the fortitude of the engineers!

When the day came to open the roundabouts, everyone was waiting to see what would happen.  And guess what did happen?

 Nothing!  That's right.  For years, this interchange would frequently back up but for the first time that anyone could remember the traffic flowed smoothly.  It was an engineering success story.

After that, roundabouts were proposed at many locations in Colorado.  The City of Colorado Springs placed several roundabouts along Lake Avenue.  These were highly successful, but there was concern over the potential traffic accidents.

In 1998, I was the conference chairman for the Colorado chapter of the Institute of Traffic Engineers.  One of the issues that plagued the traffic community was what standards should be used to design and analyze these roundabouts.  We decided to bring in experts from around the world.  We invited experts from England to Australia come to Loveland, Colorado and share their experience.  It was probably the most exciting seminar I have ever been involved with.  Particularly when the two experts almost came to blows about which method to analyze the roundabouts was greater.

The Transportation Research Board also recognized the deficiency and began to fund research into all aspects of roundabout operation.  These findings have been implemented into the 2010 Highway Capacity Manual.

Bicycle Detection

One of the most challenging aspects of traffic engineering has to deal with Bicyclists.  They're not cars and they're not pedestrians.  They're kind of a special creature.  Because of the temperate climate here in Tucson, they're everywhere.  At signalized interesections bicyclists run into a real dilemma.

The old rule we learned as kids was that at an intersections cyclists should dismount and be treated as pedestrians to cross a street. But today's cyclists act more like vehicles than pedestrians.  In fact, legally they are treated as vehicles.  When they arrive at an intersection, they should wait for the green signal indication and then cross.

But loop detectors aren't typically set for cyclists.  Some agencies have overcome this issue by adding cyclist push buttons.  But that is a rather expensive proposition.  So in some instances, cyclists have learned that they will not get a green indication.  They will either wait for another vehicle to trigger the phase, or they will ignore the signal and enter the intersection on red.

This works well, except if the police see them, they will likely get a ticket.  And no amount of explaining will help.  Let's face it.  Roadways are really designed for cars - not bicycles.  Cyclists may use the road, but they are at a serious disadvantage - especially when they collide with a vehicle.  In fact, after years of studying accident statistics, I personally will not ride a bicycle on a street.  All it takes is one errant vehicle and the results are catastrophic.

In Denver, they have solved this problem by creating separate bike lanes apart from the traffic lanes.  In fact, where I lived in Centennial, Colorado, you could ride your bike for miles and miles going underneath roadways and never even have an at-grade crossing with the street.

So I have mixed emotions concering a recent report about a new law  in Chicago.  In 2012, cyclists will be allowed to cross an intersection on the red phase if they have waited enough time. In one sense, I am grateful that police officers will no longer give them tickets.  But on the other hand, I am concerned that the law will mask a real problem with the transportation system - namely that it is really not designed to handle cyclists.

One would think that with all the technology we have and the skill in planning that we would be able to create a separate transportation system for cyclists.  In fact, I would ride a lot more if it was safer.  But I guess we need to be content with the fact that cyclists won't be ticketed for trying to live within the existing system.