U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590
202-366-4000
Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations
| REPORT |
| This report is an archived publication and may contain dated technical, contact, and link information |
|
 |
| Publication Number: FHWA-HRT-14-020 Date: January 2015 |
Publication Number: FHWA-HRT-14-020 Date: January 2015 |
DSFS systems consist of a speed-measuring device, which may include loop detectors or radar, and a message sign, which provides feedback to drivers who are exceeding a set speed threshold. The feedback may be the driver's actual speed, another message such as SLOW DOWN, or activation of warning devices such as beacons or a curve warning sign. The sign is dynamic in the sense that it interacts with each vehicle based on that vehicle's speed.
In addition to the methods used to select the DSFS system for a given site, this chapter also discusses the selection of speed thresholds and sign placement.
While it would have been possible to create a DSFS system specifically for this project, for practical reasons it was decided to select and use one or more of the many commercially available DSFS systems. The team researched DSFS systems through the Internet, displays at conferences, and existing contacts.
When selecting DSFS systems, the team focused on the type of feedback provided by the variable message sign; therefore, this chapter generally refers to sign selection rather than DSFS system selection. The most common variable message sign simply displays a vehicle's speed when it is exceeding a set threshold. The sign can also activate a flashing beacon when the speed threshold is exceeded. Several signs can also display a static message. Common messages include SLOW DOWN or TOO FAST. More complex signs allow programming of a message, with the message being limited only by the number of alphanumeric characters that can be displayed on the sign.
To select final DSFS systems, the research team developed a set of minimum criteria:
Other desirable characteristics included the following:
After this list of required and desirable characteristics was prepared, the research team contacted vendors for specifications, application guidelines, and costs. The information provided was used to develop a matrix that ranked signs by required or desirable criteria.
Signs that met the minimum criteria were categorized by the sign message type. No DSFS systems were identified that had only a flashing beacon. Figure 41 shows signs that were categorized into one of four categories.
Figure 41. Photo. Types of dynamic speed-activated feedback signs.
The various sign categories had the capability to present a number of message types. In some cases, several vendors were available for a specific sign category, and for some categories a single vendor was available.
All signs in Category 1 display a static SPEED LIMIT with the vehicle speed (XX). Category 2 can display a static YOUR SPEED and then either the speed (XX) or SLOW DOWN. Category 3 can display the same messages as categories 1 and 2 with the added function of being able to display YOUR SPEED and SPEED LIMIT dynamically, rather than statically, as well as the ability to display TOO FAST. Category 4 signs can display the same messages as categories 1, 2, and 3, but can also be programmed to display any alphanumeric message that fits within two lines and has five or fewer characters per line. Category 4 can display a curve warning symbol with the text SLOW DOWN or TOO FAST.
One or several messages could be combined to create a particular message type. A number of different message types were available depending on the sign category. Given the range of messages that could be displayed by the four categories of signs, a large number of message types could be considered. Category 4 signs can display virtually any message that has one or two lines of text each with five characters or fewer. The team decided to use message types that had been considered in other studies or used terminology that would be familiar to drivers. The team determined that the following messages fit that description and considered message types that were combinations of the following messages:
Because several message types were available, the team debated whether to test only one message type, such as SPEED LIMIT XX followed by SLOW DOWN, or whether to test several message types. Message types would be assigned to an equal number of signs. So if four message types were selected, about five sites would have been assigned each message type.
The main advantage of having a single message type is sample size for both the speed and crash evaluation. Only 20 to 24 signs were to be installed, so each message type selected would decrease the available number of samples. Multiple message types would increase the number of factors that had to be considered in the crash analysis and would decrease the number of available samples within each sign message cohort. As a result, having too many messages would affect the ability to detect whether the signs were effective overall and whether a particular message was effective.
The main disadvantage to having a single message type is that if it proved not to be effective, it would be difficult to determine whether DSFS systems on curves in general are ineffective or whether the particular message type was ineffective.
The main advantage of testing two or more message types is that it would be possible to determine whether a particular message was more effective than others and would reduce the probability that study results were influenced by selection of the wrong message type. Another advantage is that one message type might not be appropriate for every situation. The main disadvantage of multiple message types is the decrease in sample size, as described in the previous paragraph.
The team carefully considered the available message types and the advantages and disadvantages of having several sign message types. It was decided that two message types would overcome the disadvantages of having only one message but would maximize sample size.
The first message type selected was the dynamic display of YOUR SPEED XX or SPEED LIMIT XX, with the message determined by the speed threshold. Figure 42 illustrates message type 1.
Figure 42. Illustration. Dynamic speed display.
Only sign categories 3 and 4 had the capability to display message type 1. Category 4 signs can be programmed to display this message type but are typically much more expensive than sign category 3 because they have full alphanumeric capabilities. As a result, only signs from category 3 were evaluated.
Message type 1 is hereafter referred to as "dynamic speed display." This message type is also referred to as sign type 1 because each message type had a unique vendor, and in some cases, it was easier to categorize the sites by type of sign placed than by message type.
The second message type selected displays an advance curve warning symbol (message type 2 is hereafter referred to as "dynamic curve display"). This message type has been used in Europe but has had limited application in the United States. Figure 43 shows the dynamic curve.
Figure 43. Illustration. Curve warning display.
When activated, the sign displays a standard curve warning symbol as specified by the MUTCD and SLOW DOWN. The sign also has two lights on the top and bottom of the sign that blink in alternative pattern while the curve warning symbol is displayed. The only manufacturer that produces this type of sign is Dorman Varitext. This message type is also referred to as sign type 2 for the reasons given in the preceding paragraph.
In this study, the curve warning display was configured for each site to display the appropriate advance curve warning symbol already depicted at the site (i.e., if a W1-2 sign was displayed in advance of the curve, the curve warning display would also display the W1-2 symbol when activated by a vehicle).
Both vendors provided documentation that their signs were MUTCD compliant, so it was not necessary to obtain MUTCD approval.
The MUTCD describes appropriate sign placement, in advance of the curve at a distance based on posted speed. In this study, the signs were to be placed at the PC rather than upstream, resulting in a concern that the sign placement would violate MUTCD guidelines. Follow-up conversations with the Iowa DOT and FHWA resulted in the conclusion that these signs are in "conjunction" with or "supplemental" to existing advance curve warning signage and, as a result, could be placed at the PC.
The sign selection methodology and final sign types were submitted to and approved by FHWA.
It was determined each State would receive a proportionate number of each message type and the message type would be randomly selected for each site, unless an unusual condition existed at the site that made one message type more appropriate than another.
The speed display is most likely to be effective when targeted at a selected set of drivers who are exceeding a safe speed. It becomes less effective when it is activated by a large number of drivers.
It is commonly accepted that speed displays should have an upper speed threshold above which they no longer display speed, so that drivers do not "test" their speeds against the sign and travel at unsafe speeds. For this study, it was decided that an upper speed threshold for the dynamic speed display sign would be 20 mph over the posted speed limit. The upper threshold was therefore unique for each site.
It was decided that a unique bottom threshold-the lowest speed at which the speed display would be activated-should also be selected for each site. There is no need, of course, to warn drivers who are traveling at or below the posted speed limit or curve advisory speed or even slightly above those speeds because posted and advisory speeds are not always well-determined. The challenge is to set a bottom threshold that activates the speed display only for drivers who are exceeding a safe speed.
Winnett and Wheeler (2002) addressed the bottom threshold issue when they evaluated different speed-activated warning signs, including a rural curve warning system in the United Kingdom. They also felt that the threshold for triggering the sign should be set so that it did not activate for a large percent of drivers. They determined that the threshold for triggering the sign should be set at the 50th percentile speed regardless of the advisory speed. They felt that this indicated the speed at which most drivers felt safe.
As a result, for this project the team selected the 50th percentile speed for the site for the direction of travel toward the sign as the lower speed threshold for activating the speed display. The 50th percentile speed was determined during the "before" data collection. The methodology for collection and analysis of speed data before and after installation of the signs is discussed in chapter 6.
Based on the upper and lower speed thresholds, the sign face for the speed display shows the following for each situation:
Based on the upper and lower speed thresholds, the sign face for the curve warning display shows the following for each situation:
The sign message for both signs activates when a vehicle is detected traveling over the lower speed threshold. The corresponding message is displayed as long as the radar unit in the sign detects that the vehicle is still traveling over the designated threshold.
