In our last post we reviewed “Mitigating Work Zone Safety and Mobility Challenges Through ITS” (Report # FHWA-HOP-14-007) and looked at the use of “commercial off-the-shelf” systems to mitigate work zone traffic impacts. Queue systems are the most common example. But these systems also include travel time, dynamic merge and trucks entering/exiting systems.
This report also compared these with the use of custom systems and the use of permanent ITS either alone or supplemented with portable devices. As work zone ITS becomes more “plug & play” this will become much easier to do. And as it becomes easier, it also becomes the more cost effective option.
First, let’s consider off-the-shelf versus custom systems. A few short years ago all systems were, in effect, custom systems. But as time went on, common applications emerged. Dynamic merge was probably the first off-the-shelf system. This morphed into a queue warning system a short time later. Clearly, if a standard system does what you need it to do, there is no reason to consider custom systems.
But when you need something a little different, a custom system is still a good option. This study uses as an example a work zone delay performance measurement system. They used a variety of sensors to measure performance in real time and to provide that information to the DOT and the contractor.
The cost of a custom system is based on the complexity of the system, the time needed to integrate non-standard devices, and the follow up testing. They can be expensive but often these changes can be made quickly and easily. It just depends on what you are trying to accomplish. There are at least two things to consider: 1) once you have a concept of operations, try to get multiple proposals. Every system has underlying design features that help or hinder adaptation to your special application. You will find that one is probably much better suited than the others. 2) Focus your efforts and the system design on your primary goals for the project. Don’t try to do too much. Costs increase sharply with complexity while the likelihood of success falls.
Permanent ITS can sometimes give you all you need to monitor and mitigate work zone impacts. Cameras, sensors and dynamic message signs are often located in and around work zones, especially in urban areas. But two conditions must be met for them to be used for a work zone: 1) They must remain operational throughout the project. Sensors, in particular, can be a problem, and 2) they must be located at advantageous locations. A message board in the middle of the job isn’t much use in warning drivers about delays.
The use of permanent systems also depends on the goals of the deployment. If your goal is to reduce end of queue crashes, you will need sensors spaced close together. Most portable systems space sensors no more than a mile apart and often as little as a half mile apart to recognize queuing quickly and warn drivers upstream. There are locations with that number of side fire radar sensors, but they are rare.
The sensor network used for work zone impact mitigation must also fit the work zone. Much has been written recently about the use of cell phone travel time data but, at least for now, that data looks at road segments that only occasionally match up with work zones. You can monitor travel time through a corridor, but you won’t know where in that corridor (and work zone) a problem has developed.
This does not make these permanent devices useless. As this study points out, combinations of permanent and temporary devices can be used in combination to meet your requirements. It’s simply a matter of understanding the advantages and disadvantages of each.