Evaluation of Lane Reduction 

and Late Merge Signing

FHWA Publication No.: FHWA-HRT-23-070

FHWA Contact: Jesse Eisert, HRSO-30, 

ORCID: 0000-0003-0116-5141, Office of Safety Research 

and Development, (202) 493-3284, jesse.eisert@dot.gov

INTRODUCTION

When approaching a lane reduction, drivers typically attempt to merge 

into the appropriate lane as soon as possible, even in congested conditions. 

Aggressive driving can occur when drivers merge at the last minute because 

they are potentially perceived to be “cutting the line.” This perception could 

stem from traditional traffic control device messaging suggesting that the 

merging responsibilities differ by lane. For this reason, some transportation 

agencies have considered or used methods to encourage a late merge at a 

lane reduction, sometimes referred to as a “zipper merge,” in which drivers 

are encouraged to remain in their lane until the merge point and then 

alternate merging into one lane. While findings have shown the potential 

benefits of late merges, particularly during times of congestion, signing 

for these locations varies greatly across the country, with some agencies 

developing unique signs they think may be effective (Vaughan et al. 2018; 

Minnesota Department of Transportation 2008; McCoy and Pesti 2001; 

Lammers et al. 2017; Feldblum 2005; Idewu and Ishola 2009). Practices vary 

widely for both the number and placement of signs used on the approach 

where a late merge is encouraged at a lane reduction (hereafter referred 

to as a “late merge”). The degree of instruction provided through the sign 

messaging is different between agencies, with some agencies providing 

minimal instruction to drivers and others providing very detailed messages.

Because agencies provide many different options for implementing varying 

signing, little information or research on the effectiveness of these signs 

on driver behavior is available. Some documentation of piloted late merges 

is available at sites in North Carolina, Kentucky, Virginia, and Minnesota, 

among others; however, much of the research pertains to managed situations, 

such as work zones when sign messages can be changed based on traffic 

flow conditions (Vaughan et al. 2018; Lammers et al. 2017; Beacher, 

Fontaine, and Garber 2004; Minnesota Department of Transportation 2008). 

Additionally, most of these pilot locations have been on freeways and 

not on arterials. Therefore, a detailed study to investigate various sign 

designs, identify designs that are best understood and most likely to 

achieve the desired motorist behavior, and promote uniform deployment 

of signs is needed before they can be widely used in a consistent manner 

at static/unmanaged locations in the field and included in the Manual on 

Uniform Traffic Control Devices for Streets and Highways (MUTCD) 

(FHWA 2009). The present research project was carried out under the Traffic 

Control Devices Pooled Fund Study (TCD PFS). The members of the TCD 

PFS determined that this research effort should focus on arterials due to the 

limited research and information on late merge signs used on arterials.

TECHBRIEF

Research, Development, 

and Technology

Turner-Fairbank Highway 

Research Center

6300 Georgetown Pike

McLean, VA 22101-2296

https://highways.dot.gov/research
2

OBJECTIVE

This research project explores and evaluates the 

effectiveness of different static late merge signing for 

use on arterials.

APPROACH

The research team collected data through a laboratory 

and a field study. The laboratory study assessed the 

comprehension of the different sign alternatives and 

determined which sign series to evaluate in the field. The 

field study evaluated a single sign series at 12 different 

sites across 3 States to evaluate driver behavior in 

response to the signs.

Before beginning the laboratory study, the research team 

conducted a literature and state‐of‐the‐practice review to 

identify methods for signing late merge locations. As a 

part of the state‐of‐the‐practice review, the research team 

consulted with the TCD PFS members who were part of 

the project panel to determine what signing approaches 

were being used in their States. The TCD PFS members 

provided feedback on which signs they thought were 

effective and should be included in the study.

The research team identified five late merge sign series 

alternatives for potential use on arterial roads with a 

lane reduction after a signalized intersection where a 

right lane ends, which was the roadway configuration 

used throughout the project. The sign series alternatives 

were selected based on signs proposed in the Notice 

of Proposed Amendments (NPA) for the 11th edition 

of the MUTCD and on signs that are implemented in 

current State practices and/or have shown success in 

other evaluations (FHWA 2020; Colorado Department of 

Transportation n.d.; Beacher, Fontaine, and Garber 2004; 

Figure 1. Graphic. General location of each sign 

within the sign series. 

Source: FHWA.

Table 1. Sign series alternatives.

Zipper Merge 

Sign Series 1st Sign in Series 2nd Sign in Series 3rd Sign in Series

Alternative 1

Alternative 2

Minnesota Department of Transportation 2008; 

Vaughan et al. 2018; Zafian 2018; Feldblum 2005; Idewu 

and Ishola 2009). Each sign series consisted of three 

signs with the following placement: in advance of the 

intersection (first sign), downstream of the intersection 

(second sign), or at the merge point (third sign). Figure 1

shows the general placement or location of each sign in 

the series. Table 1 shows the signs included in each of 

the five sign series alternatives.
3

As shown in Table 1, the second and third signs in sign 

series alternatives 1 and 2, respectively, are the Lane 

Ends (W4‐2) symbol sign included in the MUTCD; the 

second sign in sign series alternative 2 is the “LANE 

ENDS MERGE LEFT” (W9‐2) sign included in the 

MUTCD (FHWA 2009). The other signs are new 

concepts evaluated in this study.

LABORATORY STUDY

Method

Due to the coronavirus pandemic, the laboratory study 

was conducted online to avoid in‐person testing of 

participants. One hundred participants were recruited 

through online advertising and took part in the virtual 

laboratory study. Of the 100 participants, 53 were female 

and 47 were male. Participants ranged in age from 

18 to 78 yr old (mean = 44 yr old). Participants were 

from 17 different States, with the majority located in 

Virginia (36.3 percent), Maryland (19.8 percent), North 

Carolina (12.1 percent), Connecticut (6.6 percent), 

and Pennsylvania (5.5 percent). Participants joined a 

virtual conference call with a researcher who shared 

the screen with the participants to conduct the study. 

The laboratory data collection consisted of three 

sections: comprehension testing, subjective rankings, 

and sign series creation. Each of these data collections 

is described in the following sections (comprehension, 

rankings, and create‐a‐sign series).

Comprehension

For the comprehension testing, participants viewed 

video‐based simulations of the sign series alternatives 

as if they were driving toward them on a roadway. The 

videos paused at each sign, with the sign still in view, 

while participants were asked a series of multiple‐choice 

questions. At each of the three signs, participants were 

asked, “Based on the sign(s) you’ve seen so far, will 

you/do you need to merge?” and “Do you need to merge 

right now?” At the second and third signs participants 

were also asked a third question: “Do you have the 

right‐of‐way?”

Two videos were developed for each of the five sign 

series alternatives: one from the perspective of a driver 

in the right lane, and another from the perspective of 

a driver in the left lane. Participants were exposed 

to two sign series alternatives randomized within six 

distractor sign scenarios that were also presented as 

video‐based simulations.

Table 1. Sign series alternatives. (Continued)

Zipper Merge 

Sign Series 1st Sign in Series 2nd Sign in Series 3rd Sign in Series

Alternative 3

Alternative 4

Alternative 5
4

Rankings

After completing the comprehension testing, participants 

viewed a brief video explaining the concept of a late 

merge and were given the opportunity to ask questions. 

Participants were then asked to rank the late merge 

signs on how well they thought the signs would work 

to convey the late merge concept. Participants were told 

that the signs were designed as a three‐sign series and 

were shown the general intended placement of each sign. 

Participants were given the first sign in each of the five 

sign series alternatives and asked to rank them based on 

how well they would work in sign location one (figure 1). 

They continued this process for the second and third 

sign locations using the second signs and third signs, 

respectively, in each series to provide rankings.

Create-a-Sign-Series

Finally, participants were asked to create their ideal 

three‐sign series for conveying a late merge. They were 

given a sign bank for each of the three sign locations and 

selected a series of three signs that would work best to 

convey the late merge concept.

Data Analysis and Results

The comprehension portion of the laboratory study 

provided more objective information on participants’ 

understanding of the intended sign messages, whereas 

the rankings and create‐a‐sign‐series portions provided 

participants’ subjective preference for signs and a 

series of signs. As such, the research team prioritized 

the comprehension results when considering what sign 

alternatives warranted further consideration for field 

testing. Subjective rankings and create‐a‐sign‐series 

results supplemented the comprehension results.

The following sections report response percentages 

for comprehension questions, and means and standard 

deviations for rankings.

Comprehension

The following information describes the results for each 

comprehension question the participants were asked.

Question 1: Based on the sign(s) you have seen so far, will 

you/do you need to merge?

Figure 2 shows the participant responses to question 1 by 

sign location and sign alternative. For this question, the 

research team looked at responses to determine whether 

the signs indicated to participants that they would need 

to participate in a merge. Ideally, participants would 

respond “yes” to this question because participants would 

be actively participating in the merge action regardless 

of whether they were in the lane that was ending if the 

concept of the late merge, i.e., that vehicles should take 

turns and alternate merging into one lane, was clear.

Participants viewing the signs from the right‐lane 

perspective (i.e., the lane that was ending) were more 

likely to respond “yes” or “not sure” to question 1, 

while participants viewing the signs from the left‐lane 

Figure 2. Graphic. Participant responses to comprehension question 1: “Based on the sign(s) you have seen so far, 

will you/do you need to merge?”

Source: FHWA.
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Figure 3. Graphic. Participant responses to comprehension question 2: “Do you need to merge right now?”

Source: FHWA.

perspective were more likely to respond “no” to question 1, 

but these differences were not statistically significant. 

For the first sign location, participants were between 

2.9‐ and 8.4‐times more likely to respond “yes,” 

(indicating that they would need to merge) when viewing 

sign alternatives 1, 3, 4, or 5 compared to when they 

were viewing sign alternative 2. For the second sign 

location, participants were more likely to respond “no” 

(indicating that they would not need to merge) when 

viewing sign alternatives 1 or 2. This response suggests 

that sign alternatives 1 and 2 are less likely to convey to 

participants that they would need to merge at some point, 

regardless of lane perspective (right lane or left lane).

Question 2: Do you need to merge right now?

Figure 3 shows the participant responses to question 2 

by sign location and alternative. A goal of the late merge 

is for drivers to use all lanes until the merge point, so 

participant responses would ideally indicate that they 

understood that there was not a need to merge until they 

reached the merge point, sign location 3.

Participants viewing the signs from the right‐lane 

perspective (i.e., the lane that was ending) were more 

likely to respond “yes” or “not sure” to question 2, 

while participants viewing the signs from the left‐lane 

perspective were more likely to respond “no” to 

question 2, although these differences were not 

statistically significant.

For all sign alternatives, the number of participants 

indicating that they needed to merge right increased 

as they progressed to each sign location. Participants 

viewing alternatives 3 and 5 were more likely, at all sign 

locations, to indicate that they were not sure whether they 

had to merge right now, although these differences were 

not statistically significant.

Question 3: Do you have the right-of-way?

Question 3 was only asked at sign locations 2 and 3. 

Figure 4 shows the participant responses to Question 3 by 

sign location and alternative. A goal of the late merge is 

for drivers to alternate the merge, or to take turns merging 

into one lane. Ideally, participants would respond to the 

question with “no, each lane should take turns merging 

into one lane.”

Participants viewing the signs from the left‐lane 

perspective (the lane that was not ending) were more 

likely to respond “yes,” indicating that they thought they 

had the right‐of‐way, and this difference was significant 

for sign alternatives 1 and 2. Regardless of lane 

perspective, participants were significantly more likely to 

respond “no—each lane will take turns merging into one 

lane” when they were viewing sign alternatives 3, 4, and 

5, compared to when they were viewing sign alternatives 

1 and 2. These findings suggest that alternatives 3, 4, and 

5 are more likely to convey that vehicles in both lanes 

should alternate, or take turns, merging.
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Figure 4. Graphic. Participant responses to comprehension question 3: “Do you have the right-of-way?”

Source: FHWA.

Figure 5. Graphic. Average rank (± 1 standard deviation) for all signs and positions (1 indicates the most preferred option, 

5 the least preferred).

Source: FHWA.

Based on the comprehension results, alternatives 3, 4, 

and 5 were considered further for field testing based 

on the results of the rankings and create‐a‐sign‐series 

results. These sign series alternatives better conveyed 

the eventual need to merge, the desired location 

of the merge, and the requirement for each lane to 

take turns merging into one lane. The following 

sections discuss the ranking and create‐a‐sign‐series 

results, particularly for alternatives 3, 4, and 5.

Rankings

Participants ranked signs based on how well they would 

work in each sign location (figure 1) to convey a late 

merge scenario. For each sign location, signs ranked 

the highest (best) were given a ranking of 1, and signs 

ranked the lowest (worst) were given a ranking of 

5. Figure 5 provides the results of the sign rankings 

for each location, ordering the sign alternatives from 

best to worst.
7

At position 1, alternative 5 (the “SINGLE LANE” 

sign with “BEYOND SIGNAL” plaque) was ranked 

higher than all other sign alternatives, followed closely 

by alternative 4 (the “LANES MERGE” sign with 

“BEYOND SIGNAL” plaque). Therefore, alternatives 

4 and 5 for sign location 1 were further considered for 

field testing.

There were no obvious differences between the five sign 

alternatives at position 2. Sign alternatives 1 and 2 were 

already excluded from consideration due to their low 

comprehension results. Additionally, sign alternatives 1 

and 2 are standard signs used for a typical lane drop, as 

opposed to a late merge, which could explain why they 

did not perform as well in the comprehension testing. 

Sign alternative 3 was ranked in the lowest (worst) 

position, compared to alternatives 4 and 5; therefore, the 

research team focused on sign alternatives 4 and 5 for 

further consideration.

There were no obvious differences between the 

alternatives at position 3 either. Sign alternatives 1 and 

2 were already excluded from consideration due to their 

low comprehension results. Alternative 4 was excluded 

from further consideration due to the high percentage 

(63 percent) of lower rankings (rankings of 4 or 5). 

Sign alternative 3 (“TAKE TURNS MERGE HERE”), 

and sign alternative 5 (“FORM SINGLE LANE”) were 

further considered for field testing due to rankings and 

high comprehension.

Create-a-Sign-Series

Participants were asked to create their ideal three‐sign 

series to convey the late merge concept. Figure 6 shows 

the top 10 most selected sign series, which covered 

48 percent of all participant‐created sign series.

Of the participant‐created top 10 sign series, all but 

1 included signs that were already excluded based 

on comprehension results and/or rankings. The 

create‐a‐sign‐series results suggested that participants 

tended to gravitate toward signs that they were familiar 

with, even if the comprehension results indicated 

that these signs may not have been good options for 

conveying the late merge concept.

Selection of Sign Series for Field Study

After excluding signs based first on comprehension 

results, followed by rankings and create‐a‐sign‐series 

results, there was a single sign series that remained, as 

shown in figure 6. This sign series included the first 

sign from sign series alternative 4 and the second sign 

and third sign from sign series alternative 5. However, 

for sign location 2, the comprehension and rankings 

were the same for the sign for alternative 5 and the 

Figure 6. Graphic. Top 10 3 signs series created 

by participants.

Source: FHWA.

*White boxes (not grayed out) indicate the 

sign series that was not excluded due to 

comprehension and/or ranking results.
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impacts (e.g., metering) were excluded. Additionally, 

sites ideally had no side streets or driveways between 

the intersection and the merge point. However, in 

some cases, sites were selected with a limited number 

of driveways.

The research team also considered several characteristics 

of the lane drop itself. The team prioritized sites that had 

a two‐to‐one lane drop, right‐turn impacts from cross 

streets (e.g., channelized right‐turn lanes), and lane 

drops that occurred 400–1,320 ft after the signalized 

intersection. Additionally, only sites with a right‐lane 

drop were selected for consistency. Table 2 provides a 

description of each site included in the field study.

Table 3 provides a description of the relevant pavement 

markings in place at each site, and the signing 

related to the lane drop that was in place before the 

implementation of the late merge signs.

The North Carolina sites used merge arrow pavement 

markings that were in place before and during the 

field testing. These same sites also used lane lines that 

extended through the merge, as opposed to lane lines that 

stop before the lane begins to taper. As shown in table 2, 

there was variation in the signing practices that were in 

place before the field testing. Notably, both Maryland 

sites (MD‐1 and MD‐2) had existing signing in place 

that included a “SINGLE LANE” warning sign with an 

“ALTERNATE MERGE” plaque. Furthermore, MD‐1 

did not remove the existing signing before installing the 

late merge test signs, i.e., both the existing signs and 

the test signs were in place during the data collection 

periods. At the MD‐2 site, the late merge test signs were 

duplicated (i.e., placed on both sides of the roadway). 

Test signs were also duplicated at sign location 2 of the 

NC‐4 site.

symbolic late or “zipper” merge sign from alternative 

4. Given that the symbolic late or “zipper” merge sign 

from alternative 4 was of particular interest because it is 

included in the NPA for the 11th edition of the MUTCD, 

the research team recommended a combination of the 

two signs using the alternative 4 symbol sign and adding 

the “ALTERNATE MERGE” plaque from alternative 5. 

The resulting sign series that was used for field testing is 

shown in figure 8. All three signs in this series were new 

concepts evaluated in this study and did not exist in the 

2009 MUTCD.

FIELD STUDY

Method

The field study evaluated driver behavior in response 

to the selected sign series (figure 7). The research team 

collected data at 12 different sites across 3 States: 

Maryland, North Carolina, and New Hampshire. The 

team installed cameras before the intersection and at the 

merge point to capture driver behavior before and after 

installing the late merge signs.

Right lane utilization (RLU) was the primary measure 

of sign effectiveness that was collected. The research 

team used this measure to identify the use of the lane 

being dropped at the merge point compared to the use 

of that same lane at the upstream intersection, i.e., the 

intersection before the lane drop. This measure was of 

the most interest because the goal of a late merge is 

to use both (or all) lanes up to the merge point and, in 

typical or early merge scenarios, drivers tend to merge 

out of the right lane (for a right lane drop configuration) 

as early as possible. The research team also collected 

traffic volumes and cycle failures. A cycle failure occurs 

when the light is not green long enough to allow all 

queued vehicles to pass through the intersection. Cycle 

failures provide an opportunity to look at differences 

in lane utilization upstream of the signal and merge 

behavior downstream of the signal before and after 

congestion takes place. Cycle failure is also a measure of 

how efficient the signal can run when the auxiliary lane 

is being used more often.

Site Selection and Descriptions

The research team focused on identifying sites that 

experienced significant queues, as larger traffic volumes 

and more queuing create more incentive for outside lane 

utilization and thus increase the likelihood of drivers 

alternating (zippering) the merge. Sites that experienced 

cycle failure were also ideal, as cycle failure leads to 

increased queuing. Sites with major upstream signal 

Figure 7. Graphic. Recommended sign series for 

field testing.

Source: FHWA.
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Table 2. Description of each field study test site.

Site Name State Intersection Number of 

Lanes

Lane Drop 

Distance (ft)*

Shoulder Width 

at Merge Point (ft)**

Driveways or 

Businesses Present 

on Right Side of Road

MD-1 Maryland MD 193 at 

Lottsford Road 2-to-1 lane 750 6 No

MD-2 Maryland MD 43 at 

Walther Boulevard 2-to-1 lane 1,150 12 No

NC-1 North 

Carolina NC 115 at NC 73 2-to-1 lane 1,100 3 No

NC-2 North 

Carolina

NC 54 at 

Fayetteville Road 2-to-1 lane 650 1 Yes

NC-3 North 

Carolina

NC 42 at Bratton 

Drive/Son-Lan 

Parkway

2-to-1 lane 1,000 No Shoulder No

NC-4 North 

Carolina

New Hope Road at 

US 64 Business 2-to-1 lane 1,000 1 Yes

NC-5 North 

Carolina

Falls of Neuse Road 

at Capital Boulevard 3-to-2 lanes 1,000 10 No

NC-6 North 

Carolina

US 70 Business at 

Shotwell Road 3-to-2 lanes 1,375 No Shoulder Yes

NH-1 New 

Hampshire NH-111 E at NH-28 2-to-1 lane 1,150 8 No

NH-2 New 

Hampshire NH-28 N at NH-111 2-to-1 lane 550 3 No

NH-3 New 

Hampshire

NH-111 W at 

Enterprise Drive 2-to-1 lane 400 5 No

NH-4 New 

Hampshire

NH-102 W at Winding 

Pond Road 2-to-1 lane 700 11 No

*Lane drop distances are approximate distances from the road edge of the intersection closest to the lane drop to where the 

lane begins to taper.

**Shoulder widths are approximate widths at the beginning of the taper.
10

Table 3. Existing pavement markings and signing in place at each site before the field study. 

Site Name

Merge Arrow 

Pavement Markings 

(Presence)

Lane Lines

 Pavement Markings

Existing Signing in 

Advance of Signal

Existing Signing 

Between Signal 

and Merge

Existing Signing 

at Merge Point

MD-1 No Stops before the 

lanes begin to taper N/A

“SINGLE LANE” warning 

sign with XX Feet 

(W16-2P) plaque

“SINGLE LANE” warning 

sign with “ALTERNATE 

MERGE” panel

“FORM 

SINGLE LANE” 

regulatory sign

MD-2 No Stops before the 

lanes begin to taper

“SINGLE LANE” 

warning sign 

with “BEYOND 

SIGNAL” plaque

“SINGLE LANE” warning 

sign with XX Feet 

(W16-9P) plaque

“SINGLE LANE” warning 

sign with “ALTERNATE 

MERGE” panel

“FORM 

SINGLE LANE” 

regulatory sign

NC-1 Yes Extends 

through the merge N/A

RIGHT LANE ENDS 

(W9-1) sign

Lane Ends (W4-2) sign

N/A

NC-2 Yes Extends through 

the merge N/A N/A N/A

NC-3 Yes Extends through 

the merge N/A RIGHT LANE ENDS (W9-1) sign N/A

NC-4 Yes Extends through 

the merge N/A LANE ENDS (W4-2) sign N/A

NC-5 Yes Extends through 

the merge N/A

W4-2

RIGHT LANE ENDS 

(W9-1) sign

N/A

NC-6 Yes Extends through 

the merge N/A

“LANE ENDS 2000 FT” 

regulatory sign

LANE ENDS MERGE LEFT 

(W9 2) sign

N/A

NH-1 No Stops before the 

lanes begin to taper N/A LANE ENDS MERGE LEFT

(W9 2) sign modified* N/A

NH-2 No Stops before the 

lanes begin to taper N/A LANE ENDS MERGE LEFT 

(W9 2) sign modified* N/A

NH-3 No Stops before the 

lanes begin to taper N/A RIGHT LANE ENDS (W9-1) sign

W4-2 with “RIGHT 

LANE ENDS” 

plaque

NH-4 No Stops before the 

lanes begin to taper N/A LANE ENDS (W4-2) sign N/A

N/A = Not applicable.

*Message matches the W9‐2 sign, but text sizes/layout are different.
11

Table 4. Dates ranges for observational periods used in analyses. 

Site Name Before Transition After

MD-1 5/17/22–5/19/22 5/23/22–5/27/22 7/18/22–7/20/22

MD-2 5/17/22–5/20/22 5/23/22–5/27/22 7/18/22–7/19/22

NC-1 2/3/22–2/11/22 2/14/22–2/15/22 4/13/22–4/20/22

NC-2 2/8/22–2/10/22 3/1/22–3/3/22 4/25/22–4/29/22

NC-3 2/8/22–2/10/22 3/1/22–3/3/22 4/25/22–5/5/22

NC-4 2/8/22–2/10/22 3/1/22–3/4/22 4/25/22–4/29/22

NC-5 2/8/22–2/10/22 3/1/22–3/3/22 4/25/22–5/5/22

NC-6 2/8/22–2/10/22 3/1/22–3/2/22 4/25/22–4/29/22

NH-1 11/6/21–11/9/21 11/11/21–11/12/21 3/28/22–4/1/22

NH-2 11/9/21–11/10/21 11/10/21–11/12/21 3/28/22–4/1/22

NH-3 11/8/21–11/9/21 11/11/21–11/12/21 3/28/22–4/1/22

NH-4 11/8/21–11/9/21 11/11/21–11/12/21 3/28/22–4/1/22

Observation Periods

There were three observation periods for each site. The 

“before” period observed the existing conditions (i.e., the 

signing that was already in place) before the late merge 

sign series was installed. The “transition” period occurred 

immediately after the late merge sign series was installed. 

The “after” period occurred at least 6 w after the late 

merge sign series was installed to allow time for regular 

travelers to learn the intended behavior.

Each observation period lasted at least 2 d (48 h). Data 

collection occurred continuously during these periods to 

ensure the capture of the sites’ peak periods when traffic 

volumes were typically the highest. Table 4 shows date 

ranges when data were reduced and analyzed for each 

observation period at each site.

Data Analysis and Results

The research team reduced 350 h of video footage to 

identify changes in driving behavior associated with 

the installation of the late merge signs. Reduction was 

focused on the peak periods at each site. The morning 

peaks typically occurred from 7 a.m. to 9 a.m., and the 

afternoon peaks typically occurred from 4 p.m. to 6 p.m. 

Video quality issues prevented the reliable reduction of 

signal cycles and failures, travel time (i.e., the time to 

travel between a point well upstream of the signalized 

approach (free flow) and the downstream late merge 

location), and conflicts. Each observation consisted of 

several identifiers (site, location, date, and collection 

period), the number of vehicles observed, observation 

length (minutes), and RLU. The RLU was defined as 

the proportion of all vehicles observed that occupied the 
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right lane (which is dropped at the merge point). The 

research team excluded 18 observations (5.8 percent) 

with fewer than 100 vehicles from further analysis. This 

exclusion was done to eliminate potentially unreliable 

RLU measures made on small samples and to remove 

observations with extremely low volumes.

NC‐6 was excluded from analysis because the right lane 

was being used primarily for right turns into a fast‐food 

restaurant. Additionally, right‐lane usage was discouraged 

because of shrubbery and debris partially blocking 

the lane.

The following sections provide observed and 

estimated RLU. Observed means are reported with 

standard deviation (SDs), while estimated values are 

reported with standard error (SEs). Sample sizes (N), 

F‐values (F), and p‐values (p) are also provided 

where appropriate.

Figure 8 provides the observed RLU at each site, 

location, and period. There was considerable variation 

both across and within sites. At the intersection, six sites 

(MD‐1, MD‐2, NC‐3, NC‐4, NH‐3, and NH‐4) showed 

no significant change in RLU between the three data 

collection periods. Three sites (NC‐1, NH‐1, and NH‐2) 

showed an increase in RLU from the before period to the 

after period. At the merge point, MD‐1, NC‐4, and NH‐3 

showed the anticipated trend of a small increase in RLU 

in the transition period and a larger increase in the after 

period. Other sites followed unexpected trends, such as a 

decrease in the after period (MD‐2, NC‐3, NC‐5, NH‐4), 

nonlinearities (NC‐3, NH‐1, and NH‐2), and very low 

overall RLU (NC‐1 and NC‐2).

The research team developed statistical models to estimate 

RLU by location (intersection, followed by merge 

point), data collection period (before installation, during 

a transition period soon after installation, and several 

months thereafter), volume, and roadway characteristics 

(lane drop type, shoulder width at merge, presence of 

driveways or businesses on the right side of the road, 

and the presence of merge arrow pavement markings). 

Volume was considered a potential determinant of RLU 

and quantified as vehicles per minute. The research 

team explored various ways to incorporate volume into 

statistical models. Ultimately, the best‐performing model 

(highest adjusted R2

 of 0.40) was also the simplest, with 

the raw vehicle‐per‐minutes metric and no interaction 

terms (N = 291, F = 22.1, p < 0.01).

The results indicated that RLU at the merge point 

(combined across all sites) is 12.6‐percentage‐points 

lower than at the intersection regardless of 

sign configuration (SE = 0.016, p < 0.01). 

Overall, RLU did not change during the transition period 

(difference = ‐ 0.005, SE = 0.022, p > 0.10), but increased 

by 3.3 percentage points in the after period (SE = 0.020, 

p < 0.10). Volume, lane drop distance, lane drop type, 

and shoulder width were not statistically significant. The 

presence of driveways and businesses was associated with 

an 8.7‐percentage‐point increase in RLU (SE = 0.034, 

p < 0.05). The most significant factor regarding RLU 

was the presence of merge arrow pavement markings: 

RLU was 21.4‐percentage‐points lower at sites with 

the markings compared to sites without (SE = 0.027, 

p < 0.01). Notably, all the sites that used merge arrow 

pavement markings also included lane lines that extended 

through the merge, as opposed to lane lines that stop 

before the lane begins to taper (which is what was 

present at all other sites that did not include merge arrow 

pavement markings).

Figure 8. Graphic. Observed RLU (mean ± 1 standard 

deviation) at each site, location, and time.

Source: FHWA.
13

In summary, installing the late merge signs increased RLU 

at both the intersection and the merge location in the long 

term, but not in the short term. In addition, no evidence 

that the signs increased RLU at the merge point more 

than at the intersection is available. Figure 9 shows the 

estimated RLU by location and period averaged across 

other site characteristics.

Changes in RLU varied by site. Figure 10 shows the 

estimated change in RLU from the before to the after 

period at each site. The best statistical model, however, 

indicated no difference in the change in RLU at different 

locations. In other words, RLU did not increase any more 

Figure 9. Graphic. Estimated RLU (mean ± 1 standard error) by location and period, averaged across 

other site characteristics. 

Source: FHWA.

Figure 10. Graphic. Estimated change in RLU ± 1 standard error from before to after period. 

Source: FHWA.

or less at the merge point than at the intersection. This 

analysis, therefore, does not differentiate between changes 

at the intersection and changes at the merge point. RLU 

increased by 9 to 16 percentage points (p < 0.05) at four 

sites (NH‐1, NH‐3, NC‐1, and MD‐1) and decreased by 

12 to 24 percentage points (p < 0.05) at three sites (NC‐3, 

NH‐4, and NC‐5). The remaining four sites did not exhibit 

a significant change in RLU in either direction.

Cycle Failure

Cycle failures during the data collection periods were 

minimal; therefore, the research team was unable to 

analyze this measure.
14

DISCUSSION

The research team focused on the measure of RLU 

as the goal of a late merge is to use all lanes up to the 

merge point. Overall, the results were encouraging 

considering that the late merge signs that were installed 

increased RLU at both the intersection and the merge 

point. Interestingly, the RLU increased similarly at the 

intersection as it did at the merge point, given that the first 

sign in the series (“LANES MERGE” with “BEYOND 

SIGNAL” plaque), which was the only sign in the series 

located before the intersection, does not indicate the 

specifics of the merge or that drivers should use both 

lanes. One possible explanation is that drivers familiar 

with the route on a daily or regular basis would have 

known to use the right lane after navigating the merge 

and seeing the entire sign series the first time, and thus be 

more likely to stay in their lane at the intersection the next 

time they drove the route. The presence of familiar drivers 

would also explain the findings that the late merge signs 

increased RLU in the long term (after period) but not in 

the short term (transition period); drivers who drive the 

route regularly may have needed to see the signs a couple 

of times before changing their behavior.

Considerable variation occurred across and within the 

individual sites, while RLU increased overall. This 

outcome is not necessarily unexpected, given that the 

results indicated that several of the factors that varied 

across the sites influenced RLU, including the use of 

merge arrow pavement markings and the presence of 

driveways or businesses on the right side of the road. 

Additionally, the signs and markings that were in place 

before the installation of the late merge test signs could 

also influence driver behavior in response to the late 

merge signs. For example, the Maryland sites (MD‐1 

and MD‐2) had existing signing in place that included 

a “SINGLE LANE” warning sign with “ALTERNATE 

MERGE” plaque, and these signs were left in place at 

the MD‐1 site. As depicted in figure 10, MD‐1 showed a 

significant increase in RLU, while MD‐2 did not exhibit 

a significant change in RLU. Although the existing signs 

were not included in the formal data analysis due to the 

variation in signing across sites, existing signing and 

markings could have influenced RLU at some sites.

RLU was significantly lower at the sites that included 

merge arrow pavement markings (and lane lines that 

extended through the merge), because the arrows could 

suggest to drivers in the right lane that they have different 

responsibilities in the merge than drivers in the left lane 

and/or that they should begin the merge earlier than 

intended by the late merge signs. The North Carolina 

sites were the only sites that included the merge arrow 

pavement markings, which could explain why five of 

the six North Carolina sites showed unexpected trends 

in RLU at the merge point, as depicted in figure 9, as 

follows: NC‐3 and NC‐5 showed a decrease in RLU after 

the late merge signs were installed, while NC‐1 and NC‐2 

showed very low overall RLU. The potentially perceived 

contradictions between the merge arrows and the intent 

of the late merge signs may have led to some of these 

unexpected trends in RLU for the North Carolina sites. 

Additional research could help to identify the specific 

effects of merge arrows and other pavement markings on 

merging behavior in various scenarios (e.g., when late 

merges are ideal versus when early merges are ideal).

The effect of driveways and businesses on the right 

side of the road was an interesting finding. RLU was 

8.7‐percentage‐points higher when there were driveways 

or businesses present on the right side of the road. This 

result could potentially be due to vehicles entering 

the right lane of the roadway from the driveways or 

businesses. Depending on the distance from the driveways 

or businesses to the merge point, the opportunity to move 

into the adjacent lane before the merge point might have 

been diminished, potentially leading to a higher RLU at 

these locations.

Similar to the findings of the literature and 

state‐of‐the‐practice review, this study found that the 

effectiveness of late merges tend to vary across locations. 

Several researchers and agencies have cited other factors, 

in addition to signing, that could influence the success of 

the late merge, such as the presence of trucks and shoulder 

width (Lammers et al. 2017; Vaughan et al. 2018). 

Although shoulder width was not statistically significant 

in the present study, future research that controls for 

some of the other roadway characteristics (e.g., presence 

of businesses and driveways, pavement markings, etc.) 

could be used to better determine the specific effects of 

shoulder width in late merge scenarios. Although public 

outreach was not a component of the present research 

effort, previous efforts have found both signing and 

public outreach to be crucial factors in contributing 

to the success of a late merge (Vaughan et al. 2018). 

Vaughan et al. (2018) indicated that outreach through 

press releases and other means are vital to the success of 

the late merge and should be very specific, instructing 

drivers precisely where to merge and not to merge earlier 

than prompted. Lammers et al. (2017) indicated that 

public awareness is one of the most important aspects 

of implementing a late merge because drivers must 

understand how a late merge works and that merging in 

this manner benefits them.

Drivers commonly think it is impolite to continue 

driving in the lane that is ending while the drivers who 

have merged early (i.e., moved into the slower moving 

adjacent lane in anticipation of the lane drop) are viewed 
15

as doing the appropriate thing because they are patiently 

waiting their turn. Due to this mindset, and the relative 

unfamiliarity of the late merge concept, drivers are 

less likely to change their behavior unless the desired 

actions are clearly conveyed, and they see other drivers 

participating in the late merge as well.

In addition to a public outreach campaign, the research 

team believes that there might be a benefit to using a 

supplemental sign before the intersection that indicates 

to drivers to “USE BOTH LANES.” Although a “USE 

BOTH LANES DURING BACKUPS” sign was evaluated 

in the laboratory study (alternative 2, sign location 1), 

it did not do well to convey to participants (either in 

the right‐lane or left‐lane perspectives) that they would 

need to merge at some point. Additionally, this sign was 

not ranked as highly as the other sign alternatives for 

sign location 1. Many participants indicated that they 

were confused by the “during backups” portion of the 

sign message and did not necessarily understand what 

this meant. The sign series that was examined in this 

study resulted in an overall increase in RLU. However, 

effectiveness might be further increased by supplementing 

this sign series with a “USE BOTH LANES” sign, in 

conjunction with a public outreach campaign, to better 

convey to drivers that using all lanes until the merge point 

is both acceptable and preferable.

CONCLUSION

The late merge signs that were installed in this study were 

effective in increasing RLU at both the intersection and 

the merge point. The presence of familiar drivers is a 

potential explanation for the RLU increasing similarly at 

the intersection and at the merge point, and for the RLU 

increasing in the long term (after period) but not in the 

short term (transition period), although the presence of 

familiar drivers was not evaluated by this research effort.

Several factors, in addition to the late merge signs, 

influenced RLU, including merge arrow pavement 

markings and the presence of driveways and businesses on 

the right side of the road. RLU was significantly lower at 

sites where merge arrow pavement markings were present, 

which could have been due to the potentially perceived 

contradictions between the merge arrows and the intent 

of the late merge signs. Additional research could be 

helpful in identifying the specific effects of merge arrows 

and other pavement markings on merging behavior, 

particularly when used in conjunction with late merge 

signs. RLU was significantly higher when there were 

driveways or businesses present on the right side of the 

roadway. This result could potentially be due to vehicles 

entering the right lane from the driveways or businesses; 

however, research would be needed to verify the specific 

behaviors of vehicles using businesses on the right side 

of the road. Finally, although the existing signs were not 

included in the formal data analysis due to the variation in 

signing across sites, it is possible that the existing signing 

and markings at the test sites could have influenced RLU.

It is possible that the effectiveness of the sign series 

evaluated in this study might be further increased by using 

a supplemental sign before the intersection that indicates 

to drivers to “USE BOTH LANES.” Future research 

would be useful in verifying whether this supplemental 

information would be effective. Future research could 

also be useful in determining the extent to which public 

outreach, used in conjunction with the sign series, would 

help convey to drivers that they should use both lanes 

until the merge point.

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Idewu, W., and A. Ishola. 2009. “Development and 

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16

Researchers—This study was conducted by Bryan Katz (ORCID: 0000-0003-2813-8098), Erin Kissner (ORCID: 

0009-0000-4507-0115), Erin Filler, and Steve Jackson of toXcel; and Christopher Vaughan and Christopher 

Cunningham of the Institute for Transportation Research and Education at North Carolina State University as part 

of VHB’s contract DTFH6116D00040.

Distribution—This TechBrief is being distributed according to a standard distribution. Direct distribution is being 

made to the FHWA divisions and Resource Center.

Availability—This TechBrief may be obtained at https://highways.dot.gov/research.

Key Words—Signs, late merge, zipper merge, late merge signs, warning signs, lane utilization, laboratory study, 

field study, comprehension, observational data collection.

Notice—This document is disseminated under the sponsorship of the U.S. Department of Transportation (USDOT) 

in the interest of information exchange. The U.S. Government assumes no liability for the use of the information 

contained in this document. The U.S. Government does not endorse products or manufacturers. Trademarks or 

manufacturers’ names appear in this TechBrief only because they are considered essential to the objective of 

the document.

Quality Assurance Statement—FHWA provides high quality information to serve Government, industry, and the 

public in a manner that promotes public understanding. Standards and policies are used to ensure and maximize 

the quality, objectivity, utility, and integrity of its information. FHWA periodically reviews quality issues and adjusts 

its programs and processes to ensure continuous quality improvement.

JULY 2023 FHWA-HRT-23-070

HRSO-30/07-23(WEB)E

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