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Publication Number: FHWA-HRT-13-047
Date: August 2013
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This report discusses the procedure used to develop, test, reduce, and analyze several SSs evaluated in a simulator study. From a list of potential topics identified by the research team as needing evaluations, the following topics were selected for the simulator study:
During the study, test signs were introduced in the simulation along freeway roadways to evaluate drivers' real-time response to the signs. The verbal instructions indicated a starting lane and a destination that the participants were to drive toward. The key recorded measures included lane position, lane change, and distance from SB for the lane change.
Topic 1 tested the understanding and use of different methods to sign for an option lane. The topic evaluated driver understanding of arrow per lane, down arrow per lane, or signing only for the exit and not the through movement. Almost all participants made the correct decision to exit or stay on the freeway; however, many unnecessary lane changes were made with each of the three SSs for those people whose SL was either the far left or the far right. Those drivers who started in the center lane and were given a through route destination were less likely to make unnecessary lane changes compared to all other conditions. The interesting finding is that drivers who started in the center lanes and were told to exit moved to the far right lane, which included an unnecessary lane change. However, drivers who started in the center lane and were given the through destination did not move to the far left lane. This may have been due to some reluctance on their part to move into the left lane, which is typically used for high-speed passing.
Topic 2 studied methods to create signs when two interstate exits were within close proximity, and a need existed to create signs for three destinations (two interchanges/exits and the through lanes). For SS 2‑B, which had an arrow-per-lane design, all participants (42) made correct lane change decisions. SS 2-C, which had a diagrammatic sign, also had many correct lane change decisions, with five or more of the seven participants in a group making the correct decision. Of the 42 participants who viewed SS 2-C, only 3 made incorrect lane change decisions. SS 2-A (multiple signs with exit only panels) did not have as favorable results (e.g., 6 of the 42 participants made incorrect lane change decisions). SS 2-A also had more of the participants wanting additional information to make a lane-change decision.
Topic 3 evaluated signs for an upcoming exit that then had a Y-split into two directions. Signing options included a split sign to explore whether it helped to maneuver drivers into the appropriate lane for the Y-split in advance of the initial exit. The split sign showed the two destinations side by side with a vertical white line separation. For SS 3-B, the split sign was used for the two advance signs and at the gore. SS 3-C only used the split sign at the gore with the two advance signs showing the destinations vertically stacked. SS 3-A used the vertical stacked format for both the two advance signs and the gore sign. The lateral location of the destination on the sign was used by the participants in making a lane-change decision. As can be seen in figure 28 and figure 29, several lane changes were made at the first appearance of the split exit sign (at SB I location for SS 3-B and at SB III location for SS 3-C). While several incorrect lane changes were made for each SS, SS 3-B, which used split exit signs at all three SB locations, had the fewest and was judged superior in comparison to the other two arrangements.
Topic 4 evaluated whether it was better to fill an advance single sign with supplemental way-finding information or to spread the information among multiple signs, including ground-mounted signs. Gore signs with advance signs at 1 mi were used to explore if sign spreading on a single bridge or on multiple bridges improves where the lane change is occurring. For most of the variations studied, SS 4-C (sign spreading across two SBs) had the most participants make the correct lane change decision, although SS 4-A (information for next exit stacked on one sign) also had many of the participants correctly making lane positioning decisions. When the destination information was spread across multiple signs on a single bridge, several participants made incorrect lane changes to the left when the instructions were to go to the second destination. These drivers may have been positioning their vehicle into the lane under the sign with their intended destination. This finding indicates that spreading information about the next exit across multiple signs on a single bridge may have unintended consequences if the SB also includes a sign for another exit that is located to the left of the preferred lane.
Topic 5 evaluated the effectiveness of sign spreading when there were many bits of information on one SB. One SS did not have sign spreading (SS 5-A), and the other SS (SS 5-B) had sign spreading across multiple SBs. The lateral position of a pull-through sign on the SB is important. SS 5-A had more unnecessary lane changes compared to SS 5-B: half of the participants with SS 5-A had unnecessary lane changes, while SS 5-B had no unnecessary lane changes. Because SS 5-A had more signs on a single SB, the sign for Davenport was farther to the left, which may have resulted in participants trying to position themselves below the Davenport sign, resulting in an unnecessary (but not incorrect) lane change.
Topic 6 evaluated driver understanding of the 2009 MUTCD left exit standards.(1) Only 1- and 0.5-mi advance signs were used to test how quickly a driver identified the left exit and changed lanes and if there was confusion on whether it was an exit only or optional exit. SS 6-A had a yellow plaque at the top left, and SS 6-B had a yellow panel at the bottom of the sign. Generally, for the two SSs tested under this topic, participants understood which side of the road the exit was located. It is unclear if this was because the participants were cued by the placement of the sign over the left lane, read the word "left" on the signs, or a combination of the two. The placement of the sign over the left lane resulted in the participants correctly avoiding moving across multiple lanes to make a right exit. However, when the participants did not need to make a left exit, they frequently moved out of the left-most lane—even though the lane was not an exit-only lane—due to personal preference. A few more of the non-exiting participants seeing SS 6-B with the yellow panel at the bottom of the sign moved out of the left-most lane (8 of 14) compared to the participants seeing SS 6-A with the yellow plaque at the top left (5 of 14). For this study, the difference between these two SSs was minimal.
Researchers were tasked with developing a tool that could aid practitioners in assessing the complexity of a freeway interchange and objectively compare it to other interchanges. The focus of such a tool was on geometric design factors and related effects on driver expectancy and driver workload. Researchers considered a variety of factors and formats, ultimately developing a spreadsheet tool in which users could enter site characteristics and receive a numerical complexity score for a given interchange. After several revisions, researchers settled on a spreadsheet tool that considers the effects of 32 weighted factors on as many as 4 approaches within a given interchange. The weights range in value from 1 to 5, and the sum of the 32 weights is 100 (see table 19). The estimated impact of each factor is given points, which, when multiplied by the weight, produces a weighted score on a 1,000-point scale for each approach and for the interchange as a whole.
To determine how well the spreadsheet tool would evaluate interchanges, the research team used the spreadsheet to review 28 existing sites in 11 States. The sites were submitted by State transportation departments on the basis of their perceived complexity. The 28 sites were divided into 4 distinct groups based on the spreadsheet scores ranging from a high of 590 to a low of 180. Sites with similar scores were in the same group and were viewed as having similar levels of complexity. Researchers tested multiple combinations of weights to develop scores for the 28 sites. While individual site scores changed as the weights changed, the final set of weights produced results similar to the rankings and groupings of the study sites determined by the research team. This indicates that for the characteristics included in this spreadsheet, the results produce a general sense of the relative complexity of the interchanges studied.
In summary, the complex interchange spreadsheet tool is a useful tool for objectively comparing the complexity of multiple interchanges and determining what characteristics contribute to that complexity. There may be other variables that could be useful additions to the factors already included, and it is possible that a different distribution of weights and threshold values may produce a reasonable set of scores that varies from those presented here; however, these scores allow the user to evaluate one or more interchanges to identify potential problems that drivers may face as they travel through those interchanges. Consideration of these issues can help practitioners identify potential countermeasures either through the use of traffic control devices or, ideally, through the use of revised designs to mitigate the site characteristics that are potentially problematic.
Topics: research, safety
Keywords: research, safety, Complex Interchanges, Signs, Simulator Study, Spreadsheet Tool
TRT Terms: research, Safety and security, Safety, Transportation safety