Communicating to motorists while they are en route on Iowa’s busiest freeway system while it is undergoing major reconstruction presented a real challenge to the Iowa Department of Transportation (IDOT). This was the state’s first major urban reconstruction project since construction of the interstate in central Iowa in the 1960s.
The project, which is located in the greater Des Moines area, has involved reconstruction of the 14-mile I-235 corridor. Work to upgrade or replace all 71 bridges, install new on- and off-ramps and widen the route by adding two to four lanes is being completed while under traffic and over a five-year period. Daily traffic volumes along sections of the freeway average 120,000 vehicles. The project will be completed in fall 2007.
Considering the dynamic nature of the planned construction activities, which would occur at different points along the corridor and change frequently, IDOT determined the project would benefit from an automated traveler information system. The goal of the system would be to provide travelers with current and reliable information so they could make informed travel decisions.
Initially, IDOT determined it would use its four-line-matrix dynamic message signs (DMS) situated on key freeway approaches. At the start of the project, the department also planned to install two additional DMS at other major approaches to the metropolitan region. The cost of the signs, including installation as part of a sign-bridge structure, was approximately $200,000 each and allowed a wide range of messages to be deployed.
In the Des Moines metro area’s highway network, major travel corridors include the freeway system and a network of major arterials across the region.
Data gap
The initial six-sign plan was effective in providing travelers information as they approached the metropolitan area. However, once inside the construction area, motorists who had not preplanned their trips had to rely primarily on detour signs and occasional traffic reports for construction updates and incident or diversion information. There was an obvious gap in effective communication with travelers, especially since past communication surveys conducted in the region suggested that motorists tended to prefer “active” notification of traveler information.
In response, IDOT determined it was imperative that the department develop a plan to provide travelers with information at other key interchanges and highway sections along the freeway and points in the metropolitan region. The department’s budget, however, prevented widespread deployment of the large, full-matrix DMS at all strategic motorist decision points.
IDOT, in partnership with its traffic management consultants, approached this challenge by examining several options. The result was a comprehensive, regional motorist information plan that included the installation of an additional 22 semipermanent, side-mount DMS, as well as five additional large, full-matrix DMS at key motorist decision points.
The 22 side-mount DMS used on this project are similar in design to the portable message boards commonly installed on trailers and used in work zones around the country. The message board itself is much smaller in size than those used on the major approaches to the region. As opposed to a three-line, 18-character-per-line matrix, the side-mount signs permit a three-line by eight-character message to be displayed. However, a full set of alphanumeric characters and a wide set of symbols are available to assist in deployment of appropriate and meaningful messages.
For cost-comparison purposes, the cost to purchase and install a side-mount DMS is approximately $35,000 versus the $200,000 for the full-size DMS. Costs vary based on sign supports and roadside preparation for the installation.
Several key issues were addressed when implementing the automated traveler information system for the I-235 project. They included:
- Providing a sustainable yet cost-effective mounting system for the signs;
- Providing power to the equipment;
- Providing communications from IDOT’s traffic management center to the signs; and
- Assuring that the signs displayed effective messages for the traveling public.
PAtch found
IDOT sought to provide more permanent and cost-effective mounting structures for the signs. DMS mounted on trailers were still effective at locations where construction activity demanded regular movement of the signs. Thirteen of the side-mount DMS were located on trailers.
For the more permanent installations (nine locations) several alternatives were studied. The department investigated the feasibility of placing the signs on the existing bridge structures using specially designed attachments. Because of the uniqueness of the bridges and various structural issues, this alternative was abandoned. After examining a number of roadside designs, IDOT employed a multiple-steel-post design for support, situating the signs outside the roadside clear zone. In some cases, fill material was added to the roadside edge to assure that the DMS would be well within the driver’s eye level and cone of vision.
Electrical power sources (120-/240-volt service) were provided at each site. Since the DMS were situated fairly close to existing interchanges or other property development, nearby power sources existed for most locations. Benefiting from the strong relationship that had been fostered between IDOT and the local utility company during the I-235 reconstruction project, IDOT was able to acquire power sources in a timely manner. Typically, IDOT walked each site with the utility company’s representative, defined the power needs and immediately prepared agreements for power at each location. Ongoing coordination efforts were maintained throughout the installation process.
Communications to the DMS from the metro area’s traffic management center was another critical issue. Speed and efficiency in the process of disseminating sign messages (especially under emergency situations) was an essential requirement.
Prior to installation of the new signs, IDOT was using dial-up, landline phone technology to communicate with its DMS. Using the dial-up system, signs received their messages one at a time. While this method was reliable, it was inefficient.
IDOT originally planned to use radio communication technology and special traffic management software to communicate with the new signs. The software was designed to allow simultaneous sending of messages to multiple DMS, an important feature in handling most major highway incidents in the area. Numerous field tests were conducted to evaluate the reliability of the radio system. The testing proved the wireless technology and software to be unreliable, so IDOT modified its plan and adopted the use of cell-phone technology. While not as fast as the wireless solution, the cell-phone technology has proven itself to be reliable. In addition, the initial costs, as well as the maintenance and operating costs of the cell-phone system, are much lower.
Writer’s block
While the cost savings for the side-mount units was evident, the message display capability first appeared limited. IDOT needed a viable approach to providing the required motorist information within the constraints of the side-mount displays. Upon further investigation, the department discovered several valuable resources offering guidelines on ways to provide meaningful motorist messages using limited space. The Federal Highway Administration’s (FHWA) guidelines (Changeable Message Sign Operations and Messaging Handbook, Pub. #FHWA-OP-03-070, August 2004), which are based on extensive human-factors studies, were employed in concert with IDOT’s own changeable message sign guide. The following key parameters, which are listed in IDOT’s guide, were closely adhered to in developing the messaging for the side-mount units.
Comprehension
When operating a portable changeable message sign (CMS), it is necessary to use messages that are readily understood by motorists. This can be accomplished by using standard sign messages included in the programmed memory of the CMS control system or those subsequently created by the department and stored in the system’s memory.
Customized messages
When the standard messages do not fulfill the communication needs, a customized message can be created.
Messages should take into account the following factors.
- No more than two displays should be used for any message cycle;
- Each display should convey a single thought;
- Messages should be as brief as possible;
- Abbreviations should be easily understood;
- The entire message cycle should be readable at least twice; and
- Messages shall not scroll horizontally or vertically across the face of the sign.
Message length is limited by reading time. A good rule of thumb is that a vehicle operator will need at least one second per short word (up to eight characters in length), excluding prepositions (for example, to, for and at), or two seconds per unit of information to read and recall a well-designed message. Message length also is limited by a driver’s mental processing capability. Studies have shown that an eight-word message with about four to eight characters per word, excluding prepositions, approaches the mental processing limits of drivers traveling at high speeds. The entire message should be legible at least twice at the prevailing vehicle operating speed.
The minimum information required for an effective message is a statement of the problem or effect of the vehicle operator’s decision and action required. In other words, the vehicle operator needs to know what to do and the reason for doing it. The location of the problem can sometimes be quite useful in a diversion decision. An attention statement also may be needed if only a certain group of drivers is affected, such as when concurrent routes are involved.
Abbreviations are acceptable, with a maximum of two per display and preferably with only one per display. According to the FHWA’s human-factors study, acceptable abbreviations are based on words that at least 85% of the driving public would understand.
Sample messages
It is not possible to list messages for all situations for which the use of CMS would be appropriate.
IDOT is very pleased with the performance of its automated motorist information system as it has been deployed throughout the many stages and changes that have occurred during I-235’s reconstruction. Both the overhead and side-mount DMS have proven very effective in conveying messages to motorists and obtaining their compliance.
These findings are supported by the public’s responses to questions regarding use and acceptance of the DMS, as revealed in five annual communications surveys conducted amongst I-235 users from the multicounty region. The surveys were conducted for IDOT by an independent survey firm. Over three-fourths of the population sampled reported using the DMS as a primary means of traveler information, a proportion that represented an increase over the preconstruction period, when minimal use of the DMS existed in the region.
While the messages displayed on the side-mount DMS are shorter and more concise, they obviously have been clear to the motoring public. Nearly 80% of the sampled population group from the most recent communications survey were aware of the messages and found them very helpful.
Based on the success and cost effectiveness of the motorist information program in Des Moines, similar programs are being planned in Iowa for the reconstruction of I-80 in Iowa City and a Mississippi River crossing incident management system for the I-74 bridge in Davenport.
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