Research, Development, and Technology

Turner-Fairbank Highway Research Center

6300 Georgetown Pike

McLean, VA 22101-2296

PUBLICATION NO. FHWA-HRT-22-107 DECEMBER 2022 

Preferred Following Distance as a 

Function of Speed—Function-Specific 

Automation (Level 1) Applications
i

FOREWORD 

The use of adaptive cruise control (ACC) is becoming more prevalent on today’s roadways, 

while the research and development in cooperative adaptive cruise control (CACC) is 

increasingly mature. These systems have the potential to increase the safety and operational 

capacity of the roadway. It is notable that the potential impact that these systems will have on 

roadways will depend largely on the specific parameters used in their implementation, and the 

extent to which drivers feel comfortable accepting those parameters. 

This report documents an experiment aimed at identifying comfortable gap distances used in 

ACC and CACC systems. This study used a variety of speeds that could be used to help guide set 

speeds for these systems. Participants drove an experimental course that included nine test 

speeds. The participants comfortable and minimally safe following gaps were recorded as they 

drove manually, and they rated their comfort level with the ACC gap distance at the same 

locations and speeds. The result was a creation of a set of comfortable and minimally safe 

following gap curves that were compared to the gap distance curve of an ACC system currently 

on the market. 

This report is of interest to transportation engineers and researchers, ACC developers, State and 

local transportation agencies, and other roadway safety professionals interested in understanding 

how ACC and CACC systems will affect drivers and roadway safety. 

Brian P. Cronin, P.E. 

Director, Office of Safety and Operations 

Research and Development 

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 report only because they are considered essential to the 

objective of the document. 

Quality Assurance Statement 

The Federal Highway Administration (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. 

Recommended citation: Federal Highway Administration, Preferred Following Distance as a 

Function of Speed—Function-Specific Automation (Level 1) Applications (Washington, DC: 

2021) https://doi.org/10.21949/1521938.
ii

TECHNICAL REPORT DOCUMENTATION PAGE 

1. Report No. 

FHWA-HRT-22-107 

2. Government Accession No. 3. Recipient’s Catalog No. 

4. Title and Subtitle 

Preferred Following Distance as a Function of Speed—

Function-Specific Automation (Level 1) Applications 

5. Report Date 

December 2022

6. Performing Organization Code: 

HRSO-30

7. Author(s) 

Starla M. Weaver (ORCID: 0000-0002-9559-8337), Szu-Fu 

Chao (ORCID: 0000-0002-2037-5200), Mafruhatul Jannat, 

(ORCID: 0000-0002-5218-3051) Brian H. Philips (ORCID: 

0000-0002-8426-0867) 

8. Performing Organization Report No. 

9. Performing Organization Name and Address 

Leidos, Inc. 

6300 Georgetown Pike 

McLean, VA 22101 

10. Work Unit No. 

11. Contract or Grant No. 

DTFH61-13-D-00024 

12. Sponsoring Agency Name and Address 

Office of Safety and Operations Research and Development 

Federal Highway Administration 

6300 Georgetown Pike 

McLean, VA 22101-2296 

13. Type of Report and Period Covered 

Technical Report 7/2018-6/2022 

14. Sponsoring Agency Code 

FHWA ITS JPO and HRSO-30

15. Supplementary Notes 

The Contracting Officer’s Representative was Brian Philips (HRSO-30). Brian Philips (ORCID: 0000-0002-8426-

0867) was also the Government Task Manager. 

16. Abstract 

The gap distances used on adaptive cruise control (ACC) and cooperative adaptive cruise control (CACC) systems 

have important implications for both driver safety and transportation operations The current study sought to 

identify comfortable or preferred following gaps under a variety of speeds that could be used to help guide set 

speeds for ACC and CACC. Participants drove an experimental course that included nine test speeds ranging from 

25 to 65 mi/h, once using ACC and once while driving manually. For each of the nine test speeds, comfortable and 

minimally safe following gaps were recorded as the participants drove manually. The participants also rated their 

comfort with the ACC gap distance at the same locations and speeds. The result was a creation of a set of 

comfortable and minimally safe following gap curves that were compared to the gap distance curve of an ACC 

system currently on the market. 

17. Key Words 

Driver assistance systems, behavioral adaptation, 

adaptive cruise control, lane keeping assist, gap 

distance 

18. Distribution Statement 

No restrictions. This document is available to the public 

through the National Technical Information Service, 

Springfield, VA 22161. 

http://www.ntis.gov 

19. Security Classif. (of this report) 

Unclassified 

20. Security Classif. (of this page) 

Unclassified 

21. No. of Pages 

26 

22. Price 

N/A 

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized.
iii

SI* (MODERN METRIC) CONVERSION FACTORS

APPROXIMATE CONVERSIONS TO SI UNITS

Symbol When You Know Multiply By To Find Symbol

LENGTH

in inches 25.4 millimeters mm

ft feet 0.305 meters m

yd yards 0.914 meters m

mi miles 1.61 kilometers km

AREA

in2 square inches 645.2 square millimeters mm2

ft2 square feet 0.093 square meters m2

yd2 square yard 0.836 square meters m2

ac acres 0.405 hectares ha

mi2 square miles 2.59 square kilometers km2

VOLUME

fl oz fluid ounces 29.57 milliliters mL

gal gallons 3.785 liters L

ft3 cubic feet 0.028 cubic meters m3

yd3 cubic yards 0.765 cubic meters m3

NOTE: volumes greater than 1,000 L shall be shown in m3

MASS

oz ounces 28.35 grams g

lb pounds 0.454 kilograms kg

T short tons (2,000 lb) 0.907 megagrams (or “metric ton”) Mg (or “t”)

TEMPERATURE (exact degrees)

°F Fahrenheit 5 (F-32)/9 Celsius °C or (F-32)/1.8

ILLUMINATION

fc foot-candles 10.76 lux lx

fl foot-Lamberts 3.426 candela/m2 cd/m2

FORCE and PRESSURE or STRESS

lbf poundforce 4.45 newtons N

lbf/in2 poundforce per square inch 6.89 kilopascals kPa

APPROXIMATE CONVERSIONS FROM SI UNITS

Symbol When You Know Multiply By To Find Symbol

LENGTH

mm millimeters 0.039 inches in

m meters 3.28 feet ft

m meters 1.09 yards yd

km kilometers 0.621 miles mi

AREA

mm2 square millimeters 0.0016 square inches in2

m2 square meters 10.764 square feet ft2

m2 square meters 1.195 square yards yd2

ha hectares 2.47 acres ac

km2 square kilometers 0.386 square miles mi2

VOLUME

mL milliliters 0.034 fluid ounces fl oz

L liters 0.264 gallons gal

m3 cubic meters 35.314 cubic feet ft3

m3 cubic meters 1.307 cubic yards yd3

MASS

g grams 0.035 ounces oz

kg kilograms 2.202 pounds lb

Mg (or “t”) megagrams (or “metric ton”) 1.103 short tons (2,000 lb) T

TEMPERATURE (exact degrees)

°C Celsius 1.8C+32 Fahrenheit °F

ILLUMINATION

lx lux 0.0929 foot-candles fc

cd/m2 candela/m2 0.2919 foot-Lamberts fl

FORCE and PRESSURE or STRESS

N newtons 2.225 poundforce lbf

kPa kilopascals 0.145 poundforce per square inch lbf/in2

*SI is the symbol for International System of Units. Appropriate rounding should be made to comply with Section 4 of ASTM E380. 

(Revised March 2003)
iv

TABLE OF CONTENTS 

INTRODUCTION......................................................................................................................... 1

METHOD ...................................................................................................................................... 3

Participants .............................................................................................................................. 3

Equipment ............................................................................................................................... 3

Design ....................................................................................................................................... 3

Procedure ................................................................................................................................. 4

Analysis .................................................................................................................................... 9

RESULTS .................................................................................................................................... 11

ACC Trial .............................................................................................................................. 11

Manual Trial.......................................................................................................................... 14

Comfortable ACC and Manual trials ................................................................................. 16

DISCUSSION .............................................................................................................................. 19

REFERENCES ............................................................................................................................ 21
v

LIST OF FIGURES 

Figure 1. Illustration. Scale for rating comfort with the current following distance. ..................... 4

Figure 2. Map. Experimental route. ................................................................................................ 5

Figure 3. Chart. ACC gap distance and time as a function of speed. ........................................... 11

Figure 4. Chart. Proportion of participants who rated the ACC gap distance as comfortable, 

too close, or too far as a function of speed. ......................................................................... 12

Figure 5. Graph. Plot of ACC comfort rating time gap versus speed curves. .............................. 13

Figure 6. Graph. Plot of ACC comfort rating time gap versus speed curves for gap 

distances 3 s or less. ............................................................................................................ 14

Figure 7. Graph. Plot of manual driving minimum safe and comfortable time gap versus 

speed curves. ....................................................................................................................... 15

Figure 8. Graph. Plot of manual driving minimum safe and comfortable time gap versus 

speed curves for gap distances 3 s or less. .......................................................................... 16

Figure 9. Graph. ACC and manual trial comfortable observations as a function of time gap 

versus speed......................................................................................................................... 17

Figure 10. Graph. ACC and manual trial comfortable observations as a function of time 

gap versus speed for gap distances 3 s or less. .................................................................... 18

LIST OF TABLES 

Table 1. Participant demographics .................................................................................................. 3

Table 2. Test route. ......................................................................................................................... 6

Table 3. Cumulative distance (mi) traveled on test segments as a function of test speed 

limit (mi/h). ........................................................................................................................... 9

Table 4. Speed in mi/h categories for binning following distance and following distance 

comfort ratings. ................................................................................................................... 10

LIST OF ABBREVIATIONS 

ACC adaptive cruise control 

CACC cooperative adaptive cruise control 

CAN car area network 

CDC Centers for Disease Control 

GPS Global Positioning System 

LOESS locally estimated scatterplot smoothing 

M mean 

OSHA Occupational Safety and Health Administration 

OHRP Office for Human Research Protections 

RMS remote monitoring system 
1

INTRODUCTION

Adaptive cruise control (ACC) is a commercially available SAE International Level 1 automated 

system that is growing in popularity. Traditional cruise control systems allow a vehicle to 

automatically maintain a selected speed set by the driver. ACC builds on this type of system in 

that when a vehicle with ACC approaches a vehicle moving slower than the selected speed, the 

vehicle with ACC uses radar or lidar sensors to automatically maintain a preselected gap 

between it and the vehicle ahead.

(1)Cooperative adaptive cruise control (CACC) builds on ACC 

by using dedicated communications to transmit data to, and receive data from, surrounding 

vehicles. This dedicated communication allows the system to respond to changes in speed and 

location of other CACC vehicles more quickly, even when the driver cannot see them.(2)

ACC is generally marketed as a convenience system that reduces stress and workload by 

relieving the driver of the need to continuously regulate vehicle speed and following distance.

(3,4)

However, these convenience benefits have potential implications for road safety. First, when in 

use, ACC keeps a vehicle driving at a consistent speed, which is a potentially valuable effect 

because increased speed variability has been associated with increased crash rates.

(5)

Additionally, ACC ensures that drivers maintain a consistent gap distance with the vehicle 

ahead. Even at its shortest setting, the gap distance maintained by ACC tends to be equal to or 

greater than that typically maintained during manual driving.

(7)

If ACC allows drivers to 

maintain a more consistent speed and greater following distance, then ACC could have positive 

effects on driver safety. CACC could further increase safety by allowing the vehicle to alter its 

speed and following distance in response to position and safety information that is directly 

delivered to the vehicle by surrounding traffic. 

ACC and especially CACC use also have the potential to influence road network operation. 

CACC has the potential to decrease congestion by reducing the size of gaps between vehicles, 

increasing string stability within a platoon of vehicles, and increasing the capacity of 

highways.

(2) Based on Monte Carlo simulations, van der Werf et al. estimated that both ACC and 

CACC use could lead to increased roadway capacity and that CACC market penetration could 

lead to quadratic increases in highway capacity that represented up to a 203-percent increase in 

capacity at full market penetration.

(7) However, these potential safety and operational advantages 

of ACC can only occur if drivers feel comfortable using the technology.

CACC and ACC automatically modulate vehicle speed to maintain a set time-based following 

distance behind a slower-moving lead vehicle. Utilizing a time-based following distance results 

in an instantaneous position-based following distance that is variable across a range of speeds. 

For example, a 1.1 s following distance is a greater positional distance at 60 mi/h than at 25 mi/h. 

The findings of Goodrich and Boer suggest that users will accept ACC and CACC more readily 

if the gap distances employed by the systems align to a user acceptance curve, adjusting time gap 

with speed to match empirically-derived gap versus speed preferences.(9) At lower speeds, 

drivers are hypothesized to perceive a close following distance (in time) as more comfortable 

than at higher speeds.

The objective of this research is to develop time gap curves that describe comfortable and 

minimally acceptable following distances over a range of speeds, both with and without ACC 
2

engaged. Specifically, we compared the comfort level and minimally acceptable following 

distances over a range of speeds rates, when driving with ACC, to those found when driving 

manually. The primary analysis evaluated time gap as a function of vehicle speed, regardless of 

road type or posted speed limit. Comfort ratings were obtained at approximately one-minute 

intervals under normal driving conditions. The study sought to suggest design guidelines for both 

ACC and CACC by identifying comfortable or preferred set distances under a variety of speeds. 

The COVID-19 pandemic lockdown posed some challenges at the end of the data collection task. 

To address those challenges, part of the experiment was conducted utilizing a remote monitoring 

system (RMS) in lieu of the experimenter and the participant sharing a vehicle cabin. The team 

also followed several cautionary steps to reduce the exposure risks, which were based on 

guidance and information from the Centers for Disease Control (CDC), Occupational Safety and 

Health Administration (OSHA), and Office for Human Research Protections (OHRP). 
3

METHOD 

PARTICIPANTS 

Twenty-six drivers from the Washington, D.C. metropolitan area participated in the study. 

However, data from two participants were later dropped due to data acquisition system failure 

for one participant and strong resistance to use of the ACC system for the other participant. As a 

result, our analysis was based on 24 participants. All participants were over the age of 18 and 

under the age of 66. Approximately equal numbers of male and female participants were 

recruited for the study. Within gender groups, approximately half the participants were 45 years 

or older. Table 1 displays the proportion of participants who were male or female divided by age 

group. All drivers had a valid driver’s license and met the following safe driving record criteria. 

• Have a minimum of 20/40 vision uncorrected or with contact lenses. 

• No DUI citations in the preceding 3 yr. 

• No more than one reported crash in the preceding 3 yr. 

• No reported crashes in the preceding yr. 

• No more than one moving violation in the preceding 2 yr. 

Table 1. Participant demographics 

Age Groups Female Male 

45 and older 6 5

Younger than 45 5 8

Total 11 13

EQUIPMENT 

Participants drove two trips in a 2012 sedan. Participants followed a secondary lead vehicle that 

was also a sedan for the duration of the experiment. The participant vehicle was equipped with 

ACC. During the experimental drive, the ACC was set at the closest following distance and a 

system for recording car area network (CAN) data including following distance. Data was 

collected at a rate of at least 1 Hz. 

DESIGN 

Each participant completed two drives. First, the participant followed the lead vehicle normally 

without cruise control. Speed was dictated by the lead vehicle (manual). Then, the participant 

followed the lead vehicle with the ACC set with a close time gap. This, following condition, 

served as a within-subject independent variable. The order in which each following condition 

was driven was counterbalanced. Speed was also manipulated within subjects and ranged from 

25 to 65 mi/h. Specifically, the nine test speeds were 25 mi/h, 30 mi/h, 35 mi/h, 40 mi/h, 45 mi/h, 
4

50 mi/h, 55 mi/h, 60 mi/h and 65 mi/h. Dependent variables of interest included gap time and 

participant comfort level. Participant comfort level was assessed on the scale of 1–5 (figure 1). 

Source: FHWA. 

Figure 1. Illustration. Scale for rating comfort with the current following distance. 

PROCEDURE 

Each session began with participants reviewing and signing the informed consent form. 

Participants were then asked to show a valid driver’s license and complete a brief visual 

screening to ensure a minimum 20/40 acuity (with correction if needed), which is the minimum 

visual acuity required to obtain a driver’s license in most States. After these preliminary 

procedures, the participant was escorted to the research vehicle where they were introduced to its 

controls and displays. 

Participants were instructed to follow the lead vehicle. If a nonlead vehicle cut in between the 

participant and the lead, the participant would then follow the cut-in vehicle as long as it 

remained in the same lane as the lead vehicle. When traffic was congested, cut-ins were more 

likely. The dependent and independent following distance variables were related to the vehicle 

directly ahead, and not necessarily the lead experimenter vehicle. When the participant vehicle 

fell behind because of a cut-in, the lead driver continued to the next planned roadway. The lead 

driver would then find a safe place to park on the side of the road, wait for the participant to 

appear, and then merge into traffic ahead of them. 

Figure 2 contains a map of the test route where the participants followed the lead vehicle. It also 

contains the estimated prompt locations. Segments marked as “transition” required the 

participants to make potentially hazardous driving maneuvers, such as changing lanes and 

entering/exiting highways. Therefore, the transition segments did not contain prompts, and the 

participants were not instructed to use ACC. In figure 2, the transition segments are marked with 

a thin black line, and the text segments are marked with a thick blue line. 
5

Original Photo © 2017 Google®. Modified by FHWA. 

Figure 2. Map. Experimental route. 

The participants started the first trial at the Turner-Fairbank Highway Research Center 

(TFHRC), McLean, Virginia. They traveled west on Dolly Madison Boulevard to Old Dominion 

Drive, then turned around and drove back on Old Dominion Drive to I–495 south. While on I–

495 south, they were asked to enter the I–495 express lanes and continue until they reached the 

Gallows Road exit, where they turned around and returned to the I–495 north express lanes. 

From the I–495 express lanes, they drove to the George Washington Parkway until they reached

Dolly Madison Boulevard and exited the parkway. They traveled on Dolly Madison Boulevard

until they reached Georgetown Pike, which they turned onto, followed immediately by a right 

turn onto Colonial Farm Drive. They then drove on Colonial Farm until just before the TFHRC 

entrance. This point marked the end of trial 1. 
6

During the drive, the lead vehicle dictated the speed by driving ±5 mi/h the posted speed limits. 

The result was the creation of nine different speed segments during each drive. Table 2 contains

detailed information on the test route segments, such as distance, posted speed limits, and test 

speed.

Table 2. Test route.

Segment Road Name Start Point End Point

Distance 

(mi)

Posted 

Speed 

Limit 

(mi/h)

Test 

Speed 

(mi/h)

1 Colonial Farm Rd. TFHRC

Georgetown 

Pike 0.768 25 25

2 Georgetown Pike

Colonial 

Farm Rd. Balls Hill Rd. 2.25 35 35

3 Balls Hill Rd.

Georgetow

n Pike

Old Dominion 

Blvd. 0.893 35 30

4 Old Dominion Dr.

Balls Hill 

Rd. Falls Run Rd. 4.35 40 40

Transition Falls Run Rd.

Old 

Dominion 

Dr.

Old Dominion 

Blvd. 0.217 n/a n/a

5 Old Dominion Dr.

Falls Run 

Rd. Balls Hill Rd. 4.35 40 45

6 Balls Hill Rd.

Old 

Dominion 

Dr.

Georgetown 

Pike 0.977 35 35

Transition I–495 inner loop

Georgetow

n Pike

Clara Barton 

Pkwy. 1.93 n/a n/a

7

Clara Barton 

Pkwy. I–495

Macarthur 

Blvd. 1.22 50 50

Transition

Clara Barton 

Pkwy.

Clara 

Barton 

Pkwy.

Macarthur 

Blvd. 0.457 n/a n/a

8 Macarthur Blvd.

Clara 

Barton 

Pkwy.

I–495 

Overpass 1.55 30 30

9 Macarthur Blvd.

I–495 

overpass

Seven Locks 

Rd. 0.951 30 25

Transition Macarthur Blvd.

Seven 

Locks Rd.

Clara Barton 

Pkwy. 0.728 n/a n/a

10

Clara Barton 

Pkwy.

Macarthur 

Blvd.

I–495 outerloop 1.05 50 50

Transition I–495 outer-loop

Clara 

Barton 

Pkwy.

I–495 outerloop express 

lanes 2.96 n/a n/a

11

I–495 outer-loop

express lanes

I–495 

outer-loop Gallows Rd 6.48 65 65
7

Segment Road Name Start Point End Point 

Distance 

(mi) 

Posted 

Speed 

Limit 

(mi/h) 

Test 

Speed 

(mi/h) 

Transition 

Gallows 

Rd./Holmes Rd. 

Gallows 

Rd. 

I–495 inner 

loop express 

lanes 1.15 n/a n/a 

12

I–495 inner-loop 

express lanes 

Gallows 

Rd. 

I–495 end of 

express lanes 6.25 65 60

Transition I–495 inner-loop 

I–495 end 

of express 

lanes GW Pkwy. 2.18 n/a n/a 

13 GW Pkwy. 

I–495 

inner-loop CIA Bridge 2.06 50 50

14 GW Pkwy. CIA Bridge 

GW Pkwy. 

Chain Bridge 

exit 1.53 50 55

Transition Chain Bridge Rd. GW Pkwy. GW Pkwy. 0.338 n/a n/a 

15 GW Pkwy. 

Chain 

Bridge Rd. CIA Bridge 1.21 50 55

Transition CIA Bridge GW Pkwy. GW Pkwy. 0.613 n/a n/a 

16 GW Pkwy. CIA Bridge 

Dolly Madison 

Blvd. 1.18 50 55

Transition 

GW Pkwy. exit 

ramp GW Pkwy. 

Dolly Madison 

Blvd. 0.131 n/a n/a 

17

Dolly Madison 

Blvd. 

GW Pkwy. 

exit ramp 

Georgetown 

Pike 0.874 45 45

Transition Georgetown Pike 

Dolly 

Madison 

Blvd. 

Colonial Farm 

Rd. 0.126 n/a n/a 

18 Colonial Farm Rd. 

Georgetow

n Pike 

GW Pkwy. 

entrance ramp 0.768 25 25

— — —

Transition 

Total 10.83 — —

— — — Segment Total 38.711 — —

— — — Trip Total 49.541 — —

—No data. 

Blvd. = Boulevard; CIA = Central Intelligence Agency; Dr. = Drive; GW = George Washington; n/a = not 

applicable; Pkwy. = Parkway; Rd. = Road. 

The participants completed the experimental route twice, once with ACC engaged and once 

while driving manually. At predesignated points along the route, the experimenter would make 

comfort and minimally acceptable speed ratings. During the ACC trial, the remote experimenter 

asked the participants, on a scale of 1–5, how comfortable they were with the following distance. 

The ratings were obtained at approximately 1-minute intervals under normal driving conditions, 

but always at the same prompt locations along the route (i.e., rating responses will be prompted 

by location rather than time). The participants reviewed the rating scale before the drive began

and provided verbal responses that were recorded by a research assistant. The time and location 
8

of participants’ rating responses were synchronized with vehicle, radar, and Global Positioning 

System (GPS) data by entering responses into a unified recording stream. 

During the manual driving trial, participants’ comfort ratings were not recorded. Instead, for half 

of the prompt locations, the experimenter flagged the following distance data at the prompt 

locations. The time gaps at these locations were assumed to be the participants’ comfortable or 

preferred following distances during manual driving. For the other half of the prompt locations, 

the experimenter prompted the drivers to approach the lead vehicle and briefly follow the lead 

vehicle at the closest following distance they felt was safe and then resume following at a 

comfortable distance. This distance was flagged as the minimally acceptable following distance. 

Thus, the minimally acceptable and comfortable following distance observations occurred at 

approximately the same prompt locations as in the ACC trials. Each of the nine speed segments 

contained two minimum safe distance observations and two comfortable observations. 

After the drive, participants completed a brief questionnaire wherein they indicated how familiar 

they were with ACC before the drive, how familiar they were with the experimental route, how 

long they have had their license, and how frequently they drove. The participants were then 

debriefed and compensated for their time. 

Table 3 shows the target test speeds used in the study along with the distance that the participants 

were expected to drive at that speed. Test speed ranged from 25 mi/h to 65 mi/h in 5 mi/h 

increments. Note that table 3 only includes the cumulative distance for test segments that contain 

prompts. Transition segments are excluded from table 3. 
9

Table 3. Cumulative distance (mi) traveled on test segments as a function of test speed limit 

(mi/h).

Speed

(mi/h)

Distance

(mi)

25 2.49

30 2.44

35 3.23

40 4.35

45 5.22

50 4.33

55 3.92

60 6.25

65 6.48

Total 38.71

Data collection occurred both before and after the COVID-19 pandemic lockdown. Data 

collection was suspended for several months when the pandemic began. When data collection 

resumed, the research team took several steps to reduce the risk of disease transmission based on 

guidance from CDC, OSHA, and OHRP. All study equipment and common touch surfaces were 

sanitized both before and after each data collection session. Participants and research personnel

wore face masks during data collection and engaged in social distancing as much as possible. 

Before the lockdown, an experimenter rode in the backseat of the participant vehicle, where they 

provided instructions and recorded participants’ responses. After the lockdown, the team utilized

an RMS in lieu of the experimenter and the participant sharing a vehicle cabin. The RMS 

leveraged three inputs to monitor the participant and vehicle: cameras, a microphone, and a 

vehicle CAN connection. This allowed the experimenter to monitor and communicate with the 

participant from a remote location.

ANALYSIS

Time gap data for each of the nine test speeds were binned according to recorded vehicle speed 

rather than posted speed limit. Thus, for both test conditions, time gaps and comfort ratings were

binned into the mi/h categories shown in table 4. This binning approach enabled several analyses 

to be conducted.
10

Table 4. Speed in mi/h categories for binning following distance and following distance 

comfort ratings. 

Test 

Speed 

(mi/h) 

Speed 

(mi/h) 

Greater 

Than 

Speed 

(mi/h) 

Less 

Than 

Or 

Equal 

To

25 22.5 27.5

30 27.5 32.5

35 32.5 37.5

40 37.5 42.5

45 42.5 47.5

50 47.5 52.5

55 52.5 57.5

60 57.5 62.5

65 62.5 67.5

70 67.5 —

—No data. 

The maximum possible number of observations per participant was 72 (36 ACC, 36 manual), but 

uncontrollable factors related to testing on public roads, such as slower moving traffic that 

prevented the participant vehicle from reaching its target speed, reduced the number of 

participant observations that were recorded. During data collection, there were 816 (Mean (M) = 

34 observations per participant) ACC trial observations and 829 (M = 34.5 observations per 

participant) manual trial observations captured from the participants. 

In the analysis, ratings with gap distances greater than 6 s were removed because they were 

deemed not informative. Removing this data reduced the data to 804 ACC observations and 816 

manual observations. In addition, the reduced data showed most of the observations had gap 

distances less than 3 s (90th percentile = 2.97 s.) Thus, an additional analysis on this majority 

group, including 761 ACC observations and 789 manual observations, was further conducted. 

For comfort ratings, few participants used the extreme ends of the rating scale. Too close ratings 

made up only 6.9 percent of total responses, whereas too far ratings made up only 0.9 percent. 

Therefore, participant responses were regrouped from five into three rating levels: Extremely too 

close and too close ratings were combined, and extremely too far and too far ratings were 

combined.
11

RESULTS 

To address the research goal, the following three scenarios were considered from the data: 

1. ACC trial comfort ratings with respect to time gap and speed. 

2. Manual trial gap distances (comfortable/minimum safe) with respect to speed.

3. “Comfortable” ACC and manual driving with respect to time gap and speed.

ACC TRIAL 

ACC is designed to keep the participant vehicle at a set distance from the vehicle ahead. The 

distance is generally based on time, such that as speed increases or decreases. The distance 

between the participant and the lead vehicle will also vary, but the following time gap will 

remain roughly the same. To test this assumption, the analysis was initiated by assessing the 

actual time and distance gaps recorded across the test speeds (figure 3). In figure 3, the ACC 

appears to shift from use of a stable gap distance to stable gap time at approximately 40 mi/h. 

That is, at speeds 40 mi/h and greater, gap time remains relatively stable at approximately 1.4 s, 

while gap distance increases as expected to maintain the stable time gap. However, at speeds less 

than 40 mi/h, gap distance remains relatively stable at approximately 24 m, while gap time 

increases with reduced speed to maintain the stable gap distance. It seems that the manufacturer 

of the ACC system used in this study used 24 m as a minimum allowable safe gap, and only 

transitioned to dictating following speed based on gap time once that distance had been 

surpassed (at approximately 40 mi/h). 

Source: FHWA. 

Figure 3. Chart. ACC gap distance and time as a function of speed. 
12

Figure 4 shows the proportion of participants who rated the gap they experienced during the 

ACC trial as comfortable, too far, and too close as a function of speed. Most of the participants 

rated the gap provided by the ACC as comfortable consistently across speeds. A small dip in 

comfort ratings was found at approximately 45 mi/h. At this speed, more participants felt that the 

set gap was too far from the lead vehicle. It is noteworthy, that even at this speed, when 

approximately one-third of the participants felt the gap distance was too far, another 15 percent 

of the participants reported that same distance to be too close. 

Source: FHWA. 

Figure 4. Chart. Proportion of participants who rated the ACC gap distance as 

comfortable, too close, or too far as a function of speed. 

Since actual gap distance varied slightly across participants, comfortable gap curves were created 

using both speed and following distances. Figure 5 contains a graph of the comfort ratings from 

the ACC trial as a function of speed (mi/h) and following distance (s). The far/too far curve 

contains 137 observations, the comfortable curve contains 584 observations, and the close/too 

close category contains 83 observations. 

Figure 6 was created under the same setting as figure 5 but based on the observations with gap 

distances less than 3 s. In this subset, the far/too far curve contains 127 observations, the 

comfortable curve contains 556 observations, and the close/too close category contains 78 

observations. 
13

Source: FHWA. 

Note: Each dot represents a data point. A local regression (locally estimated scatterplot smoothing (LOESS)) 

smoothing technique is used to create the overall patterns of following gap distance in seconds over speed. 

Figure 5. Graph. Plot of ACC comfort rating time gap versus speed curves. 
14

Source: FHWA.

Note: Each dot represents a data point. A local regression (LOESS) smoothing technique is used to create the overall 

patterns of following gap distance in seconds over speed.

Figure 6. Graph. Plot of ACC comfort rating time gap versus speed curves for gap 

distances 3 s or less.

MANUAL TRIAL

Figure 7 contains a graph of comfortable and minimum safe ratings from the manual driving trial 

as a function of speed (mi/h) and following gap distance (s). Observations were binned into 10

speed range categories. There is a total of 394 minimum safe following distance observations 

and 422 comfortable following distance observations.

Figure 8 was created under the same setting as figure 7 but based on the observations with gap 

distances less than 3 s. In this subset, there were 386 minimum safe following distance 

observations and 403 comfortable following distance observations. As displayed in figure 7 and 

figure 8, the average distance between drivers’ comfortable following distance and minimum 

safe following distance are approximately 0.39 s and 0.35 s, respectively, and that distance 

remains relatively stable across speeds.
15

Source: FHWA. 

Note: Each dot represents a data point. A local regression (LOESS) smoothing technique is used to create the overall 

patterns of following gap distance in seconds over speed. 

Figure 7. Graph. Plot of manual driving minimum safe and comfortable time gap versus 

speed curves.
16

Source: FHWA.

Note: Each dot represents a data point. A local regression (LOESS) smoothing technique is used to create the overall 

patterns of following gap distance in seconds over speed.

Figure 8. Graph. Plot of manual driving minimum safe and comfortable time gap versus

speed curves for gap distances 3 s or less.

COMFORTABLE ACC AND MANUAL TRIALS

Finally, the research team assessed the extent to which the distances rated as “comfortable” 

during the ACC trial matched the following distances recorded during the manual condition 

when participants were driving at their preferred following distance. Figure 9 contains the 

observations rated as “comfortable” for both ACC and manual trials as a function of speed (mi/h) 

and following gap distance (s). Close and far observations from the ACC trials and minimum

safe observations from the manual trials were removed in addition to any observations with a 

following distance greater than 6 s.

Figure 10 was created under the same setting as figure 9 but based on the observations with gap 

distances less than 3 s. As is displayed in figure 9 and figure 10, ACC gap distances tended to be 

rather similar to the gap distances maintained by participants when driving manually. At the 

lowest speeds (25–30 mi/h), ACC seems to select a more conservative gap distance than 

participants driving manually. The opposite pattern is seen within the 40–45 mi/h speed range, 

where the ACC maintains a slightly closer gap distance than participants driving manually. 
17

Manual drivers also selected a greater time gap than the ACC at the highest test speed (65 mi/h), 

when examining only following distances that were less than 3 s.

Source: FHWA.

Note: Each dot represents a data point. A local regression (LOESS) smoothing technique is used to create the overall 

patterns of following gap distance in seconds over speed.

Figure 9. Graph. ACC and manual trial comfortable observations as a function of time gap 

versus speed.
18

Source: FHWA.

Note: Each dot represents a data point. A local regression (LOESS) smoothing technique is used to create the overall 

patterns of following gap distance in seconds over speed.

Figure 10. Graph. ACC and manual trial comfortable observations as a function of time

gap versus speed for gap distances 3 s or less.
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DISCUSSION

The current study sought to identify comfortable or preferred following gaps under a variety of 

speeds that could be used to help guide set speeds for ACC and CACC. Participants drove an 

experimental course that included nine test speeds ranging from 25 to 65 mi/h, once using ACC 

and once while driving manually. For each of the nine test speeds, comfortable and minimally 

safe following gaps were recorded as the participants drove manually. The participants also rated 

their comfort with the ACC gap distance at the same locations and speeds. The study enabled the 

creation of a set of comfortable and minimally safe following gap curves that could be compared 

to the gap distance curve of an ACC system currently on the market. 

Gap distance can be measured using either time or distance. The size of the distance gap between 

vehicles has the potential to influence fuel economy, since short distances can reduce drag.

(9) As 

a result, it is sometimes hypothesized that freight vehicles, particularly freight vehicles using 

CACC or automated driving systems, may utilize distance gaps rather than time gaps to improve 

fuel economy on long haul drives.

(10) However, ACC and CACC systems utilized by passenger 

vehicles tend to be based on time gaps, rather than distance gaps, as time gaps tend to more 

closely mirror the following distances used by drivers.

(11)

Examination of the gap distances generated by the ACC in the current study found that the 

system used a combination of distance and time gaps, depending on the speed at which the 

vehicle was traveling. Specifically, at slower speeds (i.e., those less than 40 mi/h), the ACC 

system tended to use a set gap distance (near 24 m), whereas for speeds 40 mi/h and higher, the 

gap distance appeared to be based on a set gap time (near 1.4 s). It is likely that at slow speeds,

the physical gap distance that would be generated by maintaining the 1.4 s gap time was deemed 

too close to be safe, such that a minimum gap distance was implemented. This change in the 

ACC setting from gap time to gap distance at the low speeds could explain why the comfortable 

gap distances that manual drivers maintained at low speeds were closer than the gap distances 

maintained by the ACC. 

When asked to rate the gap distance provided by the near setting of the ACC, most of the

participants in the current study rated the gap distances maintained by the ACC system as 

comfortable, across all of the tested speeds. Even at 45 mi/h, when the proportion of participants 

who rated the ACC’s gap distance as comfortable was the lowest, more than half (56 percent) of 

participants felt comfortable with the assigned speed. The results suggest that the ACC system is 

likely to be valued and used by participants. 

When participants were not comfortable with the speed of the ACC, they tended to rate the ACC 

distance as being too far from the lead vehicle. This tendency was especially prevalent for high 

speeds (i.e., 45 mi /h and higher). This finding is consistent with previous work on gap distances. 

For example, when given a choice of a range of CACC gap settings, Nowakowski et al. found 

that drivers elected to set the gap at 0.7 or 0.6 s 80 percent of the time.

(6) Similarly, when Xiong, 

and Boyle allowed drivers with ACC to select between three gap sizes, participants selected the 

shortest gap size more often on the highway than during nonhighway driving.

(12) Drivers appear 

to be comfortable using the gap sizes established by ACC but may sometimes express a 

preference for shorter distances when driving at high speeds. 
20

The current study generated gap distance curves for both comfortable and minimally safe 

following distances. In general, the distance at which participants felt comfortable following a 

vehicle was about a third of a second farther than the minimum safe distance at which they were 

willing to follow the vehicle. Previous work has noted that preferred gap distance can be 

influenced by factors such as speed, road type, and congestion level.

(12) The current study builds 

on previous work by helping to define the boundary conditions of drivers’ comfort. The gap 

curve generated in the current study could be used to guide ACC and CACC manufacturers as 

they attempt to determine the parameters of their systems.

It is interesting to note that when ACC gaps and comfortable manual gaps were compared, the 

two curves were fairly similar, with the largest variations occurring at the extreme ends of the 

speed profile. As noted in the results section, at very low speeds, the participants tended to 

maintain gap distances that were closer to the vehicle ahead than the gap distances set by the 

ACC system. However, for the remaining speeds, the participants’ chosen gap distances were 

slightly farther away than that maintained by the ACC system. While the actual differences in 

gap size were small, the finding contrasts with previous work noting drivers’ preferences for 

shorter gap distances (particularly at high speeds) and even with the comfort level ratings 

expressed in the current study.

(7,12) However, that drivers’ subjective opinions about gap size 

may not always match their objective behavior is not surprising. Previous work has noted that a 

driver’s preferred following distance is not correlated with their ability to respond during 

emergency situations.

(11,13) The finding highlights the importance of using objective driving 

metrics when making decisions about gap distance parameter design. 

The gap distances used on ACC and CACC systems have important implications for both driver 

safety and transportation operations. ACC systems have potential safety benefits.

(4,9,12) However, 

these benefits will only occur if drivers feel comfortable utilizing the system. The gap distances 

maintained by the system can influence driver comfort, and ultimately driver use. ACC gap 

distances also have implications for transportation operations. ACC has the potential to increase

string stability within a platoon of vehicles, thereby increasing traffic flow and reducing 

emissions.

(2) However, ACC systems that use following distances that are greater than those used 

by manual drivers can still lead to increased congestion, particularly when ACC penetration rates 

increase.

(14) Therefore, ACC and CACC systems will provide their greatest benefit if they use the 

minimum following distance at which drivers’ safety and comfort can be maintained. The gap 

speed curves generated by the current study can help guide manufacturers as they strive to select 

ACC and CACC parameters that optimize both the safety and operation impacts of these systems 

on the road network. 
21

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