Work Zone ITS Needs Assessment

The Federal Highway Administration has created a wealth of resources for anyone considering the use of work zone ITS. One of the very best is the Work Zone ITS Implementation Guide published in January of last year (FHWA-HOP-14-008). This great guide helps those considering work zone ITS for the first time, seasoned veterans, and everyone in between.

One of my favorite features is the assessment chart found on page 25. I was reminded of this great tool during a workshop at ATSSA’s Traffic Expo this past February. The speaker, and I wish I could remember who it was, said he uses it every time he is unsure whether the use of a system is justified.

It considers 5 factors with a simple scoring system. Let’s go through it now.

Factor 1 – Duration of work zone
• >1 construction season (10 points)
• 4-10 months (6 points)
• <4 months; procurement and installation timeline is available prior to work starting (3 points)

It is much easier to deploy a system quickly than it once was so I am not sure this is truly a factor any more. But certainly longer projects are easier to justify than shorter ones.

Factor 2 – Impact to traffic, businesses, destinations, etc.
• Significant (10 points)
• Moderate (6 points)
• Minimal (3 points)

You really can’t argue with this one.

Factor 3 – Queuing and Delay
• > 2 miles for periods > 2 hours per day (8 to 10 points)
• 1-2 miles for periods of 1-2 hours per day (6 to 8 points)
• < 1 mile, or queue length estimates are not available but preconstruction, recurring congestion exists for periods , 1 hour per day (4 points)

The value ranges for this factor are going to vary with the agency using them. For some a one mile queue is no big deal, while for others it is a crisis. More importantly you need to score predictable but dynamic queuing higher as it is more likely to result in end of queue crashes. Other factors like sight distance issues (covered below) should also be considered.

Factor 4 – Temporal Aspects of Traffic Impacts
• Unreasonable for a time that covers more than just peak periods (10 points)
• Unreasonable during most of morning and afternoon peak periods in either direction (6 points)
• Unreasonable during most of a peak hour in either direction (3 points)
• Unpredictable, highly variable traffic volumes (1 point)

This could be the “phone call” predictor. Projects that will result in many very unhappy members of the public certainly justify the use of additional tools like work zone ITS. It shows that you care and are working to mitigate the impacts as best you can.

Factor 5 – Specific Issues Expected (0 to 3 points each based on judgment)
• Traffic Speed Variability
• Back of Queue and Other Sight Distance Issues
• High Speeds/ Chronic Speeding
• Work Zone Congestion
• Availability of Alternate Routes
• Merging Conflicts and Hazards at Work Zone Tapers
• Work Zone Hazards/ Complex Traffic Control Layout
• Frequently Changing Operating Conditions for Traffic
• Variable Work Activities (That May Benefit From Using Variable Speed Limits)
• Oversize Vehicles (Percent Heavy Vehicles > 10%)
• Construction Vehicle Entry/ Exit Speed Differential Relative to Traffic
• Data Collection for Work Zone Performance Measures
• Unusual or Unpredictable Weather Patterns

These are all important, and it is easy to imagine a situation where a combination of these alone justifies use of a system. Next to last is data collection for performance measures. This is the most cost-effective way of accomplishing that, and it is required. So I would suggest adding another 5 points or so automatically for data collection.

Once you have scored each factor, you add them up.
30 points or more means you will see significant benefit/ cost ratio from use of a system.
10 to 30 points means it may provide some benefits and should be considered to mitigate impacts.
Less than 10 points means it may not provide enough benefit to justify the cost.

This is simple, easy to use, and should help agencies justify the use of systems where applicable. You may download a copy of the chart by clicking WZITSscore. Please consider sending this to everyone who needs it.

Probe Data for Queue Warning

INRIX recently hosted a webinar on the use of probe data for things like queue warning. You can view a recorded version here: https://attendee.gotowebinar.com/recording/4107404171813882369 . But if you don’t have the time to watch it, I will fill you in.

The first thing to know is that one of the speakers was Todd Hartnett, Director of Business Development for ASTI. Todd described projects they now have in Maryland where they are using INRIX probe data to supplement their spot sensor data. Todd announced that ASTI has an agreement with INRIX and will now provide INRIX data as a value added service to their customers.

Todd says that probe data has several advantages. It is less expensive because you don’t have to send anyone into the field to deploy and maintain equipment. In fact, you don’t need equipment at all. ASTI has also found it useful in areas where there is not space to deploy a sensor trailer.

This may be the first time a work zone ITS provider has offered INRIX data as a standard part of their service. ASTI has apparently come up with a method to combine probe and spot data and to settle any conflicts between the two. But this example points out important differences between the two types of data that I thought we should discuss in more detail.

Simply put, probe data is usually better for monitoring overall system performance. It can show how the system, over all, is performing. It is simple, inexpensive, and easy to use. But it doesn’t provide much detail. Average speeds over a road segment are provided. How long that segment is, depends on the area and on how much you are paying INRIX for the service. INRIX does now offer a service that provides data for segments as short as 1 mile.

INRIX understands that for queue warning you must trigger those warnings quickly. And they feel segments one mile long are small enough to be effective. Certainly their system will help. But in my experience for a queue warning system to be truly effective, you need more:
1) You usually need even shorter distances between sensors (or shorter segments). Most of our work has spaced sensors an average of three-quarters of a mile apart. In some cases we have spaced them as little as a half mile apart. This is important in areas with poor sight distance such as horizontal or vertical curves and in areas with very dynamic queuing.
2) You must be able to set the locations where you measure those speeds. Probe data segments are not adjustable. You get what they give you. So a project might sit in the middle of a much larger segment. Or it might overlap 2 or more segments. Furthermore, every queue warning deployment we have provided involved some adjustment of sensor locations to catch the queue as early as possible, and report them in a consistent manner.

It also helps when you can add more sensors within work zones, and fewer in advance of them. In this way you will know not just that you have a problem, but where the source of that problem is in the work zone. Is it at the taper? Or at a lane shift in the work area? With this information you can immediately make traffic control changes and, perhaps, eliminate the cause of the queuing all together. With probe data, you won’t have that ability.

INRIX queue warning is a less expensive solution and I understand why ASTI chose to incorporate their data in their service. But when it comes to queue warning systems, you really do get what you pay for. Systems that use spot data are not normally expensive either, and they will warn drivers faster and more accurately than will a probe data system.