Guiding force

Safety Article June 11, 2009
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Navigating through work zones that occur within the vicinity of urban freeway interchanges can be particularly challenging to drivers. Numerous existing and temporary guide signs, short auxiliary lane segments, multiple lane exits, high merging traffic and other conditions in the work zones present complex driving situations and place considerable work load on drivers.

Sometimes work activities temporarily shift travel lanes such that existing guide signs are no longer located directly over their appropriate lanes. From a cost-effectiveness perspective, it is desirable to maintain the use of the existing guide signs as long as possible during construction rather than construct new temporary signing for each phase of the project. However, determining whether existing signs can continue to be utilized, as well as selecting the most appropriate and worthwhile method of temporarily modifying existing guide signs when necessary, is a challenge.

In order to use existing guide signs where they have been shifted out of alignment with the travel lanes (or vice versa), some agencies cover the lane-assignment arrows on the signs. Unfortunately, it is not known if removing the lane-assignment arrows, which provide path guidance information, significantly increases driver confusion as they approach decision points at the interchange. In some cases, smaller temporary-lane-assignment signs (black legend on an orange background) placed on the overhead sign gantries or route shield in-lane pavement markings in the travel lanes are used to help provide modified lane-assignment information. However, the effectiveness of these strategies has not been verified.

Recently, Texas Transportation Institute (TTI) researchers designed and conducted a driving simulator study to determine whether the location and accuracy of driver lane changes made in advance of major freeway-to-freeway interchanges were affected by the use of:

  • Temporarily misaligned permanent guide signing;
  • Temporary guide signing; and
  • Route shield in-lane pavement markings.

Researchers tested six treatments. The treatments included construction and nonconstruction conditions, lane-assignment arrow and no lane-assignment arrow conditions, properly aligned and misaligned guide signs and the use of additional devices to supplement the way-finding information (such as temporary signing, route shield in-lane pavement markings or both temporary signing and route shield in-lane pavement markings).

Call your exit

For each treatment, the subject began driving on a particular three-lane freeway. Several miles down the road, the researcher gave the subject a destination (for example, 51 north to Walker) and told the subject that they were approaching an interchange. The subject then encountered two sets of advance guide signing: one in advance of the tip of the exit-ramp gore area and one at the tip of the exit-ramp gore area.

The advance guide signing indicated the current interstate freeway and a U.S. highway number and city name. The sign panel indicating the U.S. highway was positioned on the same side as the exit lanes, consistent with the Manual on Uniform Traffic Control Devices (MUTCD) requirements. In some instances, the destination given by the researcher was the U.S. highway number, implying that the subject should exit the freeway. In other instances, the destination given by the researcher was the interstate freeway number, implying that the subject should remain on the freeway and pass the interchange without exiting.

Researchers presented both left- and right-hand exits and varied the type of exit. Researchers also manipulated which lane subjects were in as they approached an interchange (through or exit lanes).

The temporary signs were placed on the first overhead sign gantry to the right or left of existing guide signs to match the exit direction, a common practice in Texas. The route shield in-lane pavement markings appeared in the travel lanes one-third of the way between the two overhead sign gantries. The locations of these additional devices were chosen based on current Texas practice.

After each treatment the researcher asked the subject whether it was clear which lane they needed to be in to reach the specified destination; which piece of information helped them the most; and if there was any piece of information that was confusing.

Arrived

At the end of each session, each subject rated how helpful the additional devices were at reducing any confusion. In addition, for each treatment researchers computed the percent of subjects making correct and incorrect maneuvers, percent of subjects making unnecessary lane changes and the mean distance between the initiation of the final lane change and the tip of the exit-ramp gore area. Thirty-six subjects participated in the study.

All of the subjects followed the correct path; however, for each treatment approximately 30% of the subjects made unnecessary lane changes. The majority of unnecessary lane changes were subjects moving from the center lane to either the left or right lane when they could have remained in the center lane to reach their destination and subjects moving from the left lane to the right lane (across the center lane) or vice versa when they could have just moved into the center lane to reach their destination. Based on the subjects’ comments, they made these unnecessary lane changes even though they knew they were in the correct lane in order to ensure they would be able to follow the correct path (either exit or remain on the freeway).

The study determined the mean final lane change distance upstream of the tip of the exit-ramp gore area by treatment and the percent of subjects who thought the information provided was clear. As expected, treatment 1 (base condition with no construction and properly aligned guide signs with lane assignment arrows) resulted in subjects changing lanes the earliest (3,346 ft upstream of the tip of the exit-ramp gore area).

In addition, 85% of the subjects thought that treatment 1 provided clear information about the lane they should be in to reach the destination. When construction was added and the lane-assignment arrows were removed on the first set of overhead guide signs (treatment 2), the mean final lane change distance upstream of the exit-ramp gore area decreased to 2,834 ft (512 ft closer to the gore), but the percent of subjects who thought it was clear which lane they needed to be in to reach the destination remained essentially the same.

The largest reduction in the mean lane-change distance (922 ft) occurred with treatment 3, which contained construction, no lane-assignment arrows and temporary misalignment of the first set of guide signs. It also was apparent that subjects were not as sure about which lane they needed to be in to reach the destination in this situation.

Through the use of additional devices (temporary sign [treatment 4], route shield in-lane pavement markings [treatment 5], or both [treatment 6]), the mean lane-change distance upstream of the tip of the exit-ramp gore area increased to between 2,692 and 2,950 ft and the percent of subjects who thought it was clear which lane to be in to reach the destination increased to between 58 and 75%.

It should be noted that the route shield in-lane pavement markings were shown 1,171 ft downstream of the temporary sign; thus, a direct comparison between treatment 4 and treatment 5 cannot be made. Based on the subjects’ comments, the lower percentage of subjects who thought treatment 5 was clear (58%) can be attributed to the removal of the lane-assignment arrows and the temporary misalignment of the existing guide signs, not the route shield in-lane pavement markings.

After each scenario, the researcher asked each subject which piece of information helped them the most and was there any piece of information that was confusing. With treatment 1 the subjects relied heavily on the first set of overhead guide signs. Once the lane-assignment arrows were removed from the first set of signs (treatment 2) and these signs were misaligned with travel lanes (treatment 3), subjects thought these signs were confusing and depended on the second set of overhead guide signs located at the exit-ramp gore area to determine which lane they needed to be in to reach the destination.

For the treatments that included the temporary sign (treatment 4), route shield in-lane pavement markings (treatment 5) or both of these devices (treatment 6), at least half of the subjects thought these devices were helpful in determining which lane they needed to be in to reach the destination.

In addition, the use of the temporary sign in conjunction with the first set of overhead guide signs reduced the number of subjects who thought the information provided was confusing. Both devices reduced the need for subjects to wait and receive information from the second set of overhead guide signs. Even though some of the subjects stated that the additional devices were confusing, the reasons provided mainly dealt with the novelty and format of the devices.

At the end of the driving session, each subject rated on a scale from 1 to 5 (with 1 being “very helpful” and 5 being “not helpful”) how effective the temporary signs and route shield in-lane pavement markings were at reducing any confusion. The average rating across all subjects for the temporary signs and route shield in-lane pavement markings was 2.5 and 1.6 out of 5, respectively. Thus, the subjects rated the route shield in-lane pavement markings a little more helpful than the temporary signs.

A clear path

The results of this study verify that when the lane-assignment arrows on existing overhead guide signs are covered and the signs are temporarily misaligned with the travel lanes, drivers are not sure which lane they need to be in to reach their destination and thus wait to make a lane change until closer to the exit-ramp gore area.

At a more complex urban freeway interchange with higher traffic volumes, this delay may result in erratic maneuvers such as hard braking, last-minute lane changes and vehicle conflicts during merging, all of which increase the potential for crashes. Thus, additional information needs to be presented to drivers in order to provide proper and clear path guidance in advance of the interchange.

The results also indicate that smaller temporary lane-assignment signs (black legend on an orange background) placed with the overhead guide signs or route shield in-lane pavement markings in the travel lanes or both provide modified lane-assignment information. Compared with the scenario described above (misaligned guide signs with the lane-assignment arrows covered) these devices resulted in drivers making their lane change farther upstream of the exit-ramp gore area. Thus, either of these devices or a combination of these devices should be used to provide drivers with additional path guidance in work zones where the existing overhead guide signs are temporarily misaligned with the travel lanes due to work activity and the lane-assignment arrows are covered.

About the author: 
Finley is an associate research engineer at the Texas Transportation Institute (TTI), College Station, Texas. Ullman is a senior research engineer at TTI. Nelson is a research associate at TTI.
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