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Vol. 35/No. 3 August 1968

Public Roads

A JOURNAL OF HIGHWAY RESEARCH

U.S. DEPARTMENT OF TRANSPORTATION

FEDERAL HIGHWAY ADMINISTRATION BUREAU OF PUBLIC ROADS

Public Roads

A JOURNAL OF HIGHWAY RESEARCH

U.S. DEPARTMENT OF TRANSPORTATION ©

Published Bimonthly ALAN S. BOYD, Secretary Harry C. Secrest, Managing Editor @ Fran Faulkner, Editor FEDERAL HIGHWAY ADMINISTRATION Joan H. Kinbar, Assistant Editor LOWELL K. BRIDWELL, Administrator BUREAU OF PUBLIC ROADS August 1968 / Vol. 35, No. 3 F. C. TURNER, Director

THE BUREAU OF PUBLIC ROADS FEDERAL HIGHWAY ADMINISTRATION U.S. DEPARTMENT OF TRANSPORTATION Washington, D.C. 20591

FHWA REGIONAL OFFICES No. 1. 4 Normanskill Blvd., Delmar, N.Y. 12054. CONTENTS Connecticut, Maine, Massachusetts, New Hampshire, New Jersey, New York, Rhode

Jsland, Vermont, and Puerto Rico.

, ~ ; No. 2. 1633 Federal Building, 31 Hopkins Passing Aid System I, Initial Experiments, Place Baltimore. Md. Boge e

by Duke Niebur .........-. 0000. 61 Delaware, District of Columbia, Maryland,

Ohio, Pennsylvania, Virginia, and West Vir- inia. Interstate System Accident Research—Study II, No: 4 1720 Peachtree Rd., N.W., Atlanta, Ga. Interim Report II, by Julie Anna Cirillo... . 71 30309. Alabama, Florida, Georgia, Mississippi, North Carolina, South Carolina, and Tennessee. New: Pablicationvn. aa.) <- fee 2 ee ee 76 No. 4. 18209 Dixie Highway, Homewood, III. 60430. Iilinois, Indiana, Kentucky, Michigan, and Wis- consin. No. 5. Civic Center Station, Kansas City, Mo. 64106. lowa, Kansas, Minnesota, Missouri, Nebraska, North Dakota, and South Dakota. No. 6. 819 Taylor St., Fort Worth, Tex. 76102. Arkansas, Louisiana, Oklahoma and Texas. No. 7. 450 Golden Gate Ave., Box 36096, San Francisco, Calif. 94102. Arizona, California, Hawaii, and Nevada. . No. 8. 412 Mohawk Bldg., 222 SW. Morrison _ St., Portland, Oreg. 97204. , Alaska, Idaho, Montana, Oregon, and Washington.

No. 9. Denver Federal Center, Bidg. 40, Den- ver, Colo. 80225. Colorado, New Mexico, Utah, and Wyoming. No. 15. 1000 N. Glebe Rd., Arlington, Va. 22201. Eastern Federal Highway Projects No. 19. Apartado Q, San Jose, Costa Rica. Inter-American Highway: Costa Rica, Guate- mala, Nicaragua, and Panama.

COVER Public Roads, A Journal of Highway Research, is sold by the Superintendent of Documents, Government Tonite kant "y GRE Printing Office, Washington, D.C. 20402, at $1.50 per Trathe backed adap Maryland year (50 cents additional for foreign mailing) or 25 highway U.S. 301 before it be- cents per single copy. Subscriptions are available for came a dual highway. Public 1-, 2-, or 3-year periods. Free distribution is limited to

public officials actually engaged in planning or con-

Roads research is endeavoring structing highways and to instructors of highway engi-

to solve the problem of passing neering. There are no vacancies in the free list at vehicles E i , present. ;

é ; wd ond lane highways. Use of funds for printing this publication has been ap- See article beginning on op- proved by the Director of the Bureau of the Budget, posite page. March 16, 1966.

SE ee Se a ee et ed Contents of this publication may be reprinted. Mention of source is requested.

Vhen deciding whether to pass or not to ass on 2-lane highways, motorists in the uture may be assisted by electronic ystems.

Passing Aid System |

Initial Experiments

Reported by DUKE NIEBUR, Highway Research Engineer,

Economics and Requirements Division

Development of a traffic system to aid motorists in passing vehicles on 2-lane rural highways is one of the chief objectives of the Public Roads research and development program. Anyone who has driven on winding, hilly, rural roads has frequently been confronted with the problem of passing a slower vehicle ahead and has either driven many laborious miles waiting for an opportune time to pass or has ventured doubtfully into the passing maneuver on the chance that it could be accomplished without mishap. If the motorist had sufficient information about conditions on the highway ahead—whether there is an on- coming vehicle in the opposite lane and whether there is enough room on the highway to pass the car ahead and clear the oncoming vehicle—the passing maneuver not only could be executed more safely, but the volume of the traffic served by the roadway would be increased by minimizing inherent delays caused by slower vehicles.

The Public Roads research and development program has turned to electronics in the search for a method of providing information that the driver needs to pass vehicles safely on 2-lane highways. Results of experiments conducted on a 2-lane roadway with an elementary passing aid system, PAS I, are described in this article. The purpose of these experiments was to determine whether drivers would rely on information supplied electronically to indicate the absence of opposing vehicles when visual sight distance was limited. Encouraging results of the experiments, as shown by acceptance of electronically indicated passing opportunities, have prompted the planning of more advanced experiments and the development of a more sophisticated passing aid system. Work is now under- way on Passing Aid System II (PAS II), which is expected to be installed on 15 miles of 2-lane rural highway during 1969.

PUBLIC ROADS ® Vol. 35, No. 3

+z | Aen : ae

Bye de OFFICESOE RESEARCH AND DEVELOPMENT BUREAU OF PUBLIC ROADS

Introduction

RIVING on high-volume, winding, 2-lane

rural highways is a problem that is known to most motorists. Restricted sight distances, oncoming traffic, and adverse environmental conditions make it difficult or impossible for a motorist to pass slow vehicles, and any one of these conditions not only encourages unsafe passing attempts but also tends to decrease average vehicle speed. Furthermore, difficult passing situations, such as those existing on winding mountain roads and in dense traffic, discourage all passing attempts and encourage the unsafe practice of tailgating.

Less known to the layman is the decrease in the capacity of a highway caused by the inability of motorists to pass vehicles ahead. ven when passing sight distances are ade- quate, traffic volume still may reach only 30-70 percent of the roadway’s capacity (volume/capacity ratio). Unfortunately, as most 2-lane rural highways do not have unrestricted passing sight distances, the volume/capacity ratio is further reduced, and

INITIATION OF

PASSING MANEUVERS

1ST FLAG

2ND FLAG

BEGIN TEST RUN AT STATION [80 FAIRBANK HIGHWAY RESEARCH LABOR- ATORY

Figure 1.—Test route and operations for Passing Aid System, I.

the effect is to reduce the number of passing opportunities and create more traffic inter- ferences—slowdowns, accidents, ete.—which reduce the service volume.

Even after the Interstate System has been completed, more vehicle-miles will be traveled on rural highways than on rural sections of the Interstate System. From this fact alone, it is evident that rural highways must be made safer. More than one-third of all accidents on these highways at present are rear-end collisions. Head-on collisions do not oecur as frequently as rear-end collisions— about one-fifth of the accidents are head-on collisions—but they are likely to be more severe. Both types of accidents, however, involve the interaction of two or more drivers and their vehicles.

According to past research, a driver cannot estimate, with any degree of precision, the absolute speed of a vehicle ahead or the rate at which his own vehicle is approaching it until the two vehicles are only a few hundred feet apart. Also, according to past research, when the two vehicles are this close to each other, there is not enough time for the driver to modify his speed, especially if he is traveling at a speed typical of those on the highways today.

To avoid rear-end collisions, drivers need to be given reliable information about the speed of the vehicle ahead, or about relative speed or closure rate. Speed patterns of pairs of vehicles involved in rear-end collisions support the fact that the driver of the colliding vehicle lacked information on the vehicle ahead—more than one-third of the passenger cars were traveling at speed differences greater than 30 m.p.h. prior to collision. In normal traffic, however, less than 1 percent of pairs of cars travel at speed differences exceeding

30 m.p.h. Head-on collisions, although oc-

PASSING CAR

SYSTEM CONTPOL UNIT

TRAFFIC

SIGHT RESTRICTORS DETECTORS

INSTALLED FOR MEST

LEAD CAR

curring less frequently than rear-end collisions, must be given equal attention because of their severity.

Research has shown that the average driver requires approximately 9 seconds to initiate and complete a passing maneuver on a 2-lane rural highway. Thus, if one vehicle traveling at 70 miles per hour overtakes another, a 9-second passing maneuver requires that the highway ahead be clear for a distance of more than 1,800 feet. At this distance, not only are drivers unable to estimate the relative speed of a vehicle in the opposite lane but they are incapable of determining whether that vehicle is stopped, in fact, whether its motion is toward them or away from them. Many 2-lane, bidirectional rural highway sections are without sight distances of 1,800 feet and, accordingly, are marked to prohibit passing. Moreover, the degree of precision in executing the passing maneuver has become increasingly important as traffic volumes have increased and vehicles in the opposite lane are being encountered more frequently.

Public Roads, through its national program of highway research, is endeavoring to increase travel safety on rural highways by developing methods to give the driver adequate environ- ment information on 2-lane roadways. This information may relate to speed, acceleration, closure rate, or other information about both the vehicle ahead and the vehicle the opposite lane.

The objective of this research is to develop a system to aid drivers solve discrimination, judgment, information, and vehicle control problems on 2-lane rural highways, and, consequently, raise highway service volumes and increase traffic safety.

Applications of electronics technology are being explored as a means to aid drivers in making judgments and

during overtaking

WEST ENTRANCE TO FAIRBANK HIGHWAY RESEARCH STATION

OPPOSING CAR IN OPPOSING LANE

* STA.NO. | q

SIGHT RESTRICTORS INSTALLED FOR TEST

passing maneuvers. A specific application the development of an electronic aid syste that will provide the driver with informatic as to the presence, location, and speed | vehicles in the opposing lane. It was post) lated that over a specified distance of sufficie) length, considering all combinations of vehie velocities, road grades, ete., a go or no-go tyj of system could be employed.

A full-seale mockup of an electronic passil aid system has been constructed and test on the 2-lane access road to the Public Roa Fairbank Highway Research Station McLean, Va. Summarized in this article a the concepts, experimental tests, and pr cedures used to determine whether drive can and will use this electronic aid syste known as Passing Aid System I, or PAS The willingness of drivers to use PAS I, al their ability to apply it successfully as an a in passing vehicles on 2-lane highways, pl vides an indication of the advisability developing a more advanced passing @ system. .

Considerations in Developing a Passing Aid System

To speed development of passing aid § tems, tentative decisions were made: Drivers would be given distance informati and possibly speed information or time- meeting information, and (2) the system I to be compatible with existing operations that drivers of vehicles unequipped Ww electronic hardware could continue to use highway.

The following basie questions need to answered before a_ fuil-scale passing system can be made available for public 1 e Will drivers pass if they have informat

about the absence of opposing vehit

within a critical distance?

August 1968 ® PUBLIC RO.

Table 1.—Test subjects used as vehicle drivers

Distance driven during last 12 ar months Driving \ Phase Test Age experience Occupation subject 2-lane City rural Freeways streets highways Numober Years Years Miles Miles Miles 20 3 UU CON taeenee eee a 75 20 5, 500 2 20 3 SUNGeD ssser eae aoe 100 1, 500 5, 500 3 21 5 Sitdenitewns ees 500 2, 500 2, 500 4 ee 5 Studentees.2 ae 4, 000 2, 000 2, 500 5 23 7 BPR Meee ee Se 500 2, 000 2, 000 Preliminary Tests__-- 6 27 10 BPR Bngineer!--_.2- | 4, 000 7, 000 5, 000 i 22 6 Ntudenpsss eS 100 800 2, 000 8 30+ 30 pecretary == oe 8, 000 3, 000 1, 000 9 24 8 Secretary..............| 3,000 6, 000 3, 000 10 62 15 PeCralar vers ce easy 500 500 1, 000 11 23 7 IB Din Mees Ae eB || 500. 2, 000 2, 000 12 29 13 BPR Engineer______- 14, 000 6, 000 10, 000 13 22 5 MOCTOLAT Yess eee ee 5, 000 10, 000 5, 000 Experiment 1___-_..- 14 21 1 STIGSR Taree oan ee 2, 500 5, 000 2,500 15 20 3 Studentesseaq e vee | 1, 000 2, 000 2, 000 16 58 40 BPR Foreigner ______- 800 1, 700 38, 000 yee Le FY Eee eee, OL ewes piled! seal inoteghetac yb. ce, Howe ce are wy eM |anney, og Bie. guy | ag 18 35 19 BPR Engineer_____-.- 4, 000 4, 000 4, 000 19 31 10 BPR Engineer___-_-_-_-_- 3, 000 1, 000 6, 000 20 45 18 BPR Hngineer_._____- 4, 000 8, 000 1, 000 Experiment 2----.--- 21 26 11 BER Engineer 2. > | 8, 000 8, 000 2, 000 22 18 2 Studentias ewes eee eee es 38, 000 1, 000 23 51 30 Spiritualist___ yt Oo 38, 500 7, 000 3, 500 24 35 13 Hlousewites. es. eee | 4, 000 8, 000 3, 000 25 20 5 Stident# ee Se | 5, 000 1, 000 6, 000

What criteria should be employed in deter- mining the critical distance at which an opposing vehicle is brought to the attention of the passing driver?

Will drivers employ other distance informa- tion about opposing vehicles in addition to the critical distance?

/ Will drivers use opposing vehicle speed information in making a passing maneuver? Will drivers use time-to-meeting informa- tion in making a passing maneuver?

How long does it take drivers to adapt to the new system?

| What instructions should be given to drivers to make it easy for them to learn how to use the new system?

What criteria should be employed in deter- mining how far apart the vehicle detectors shall be?

Are there any side effects and reliability considerations that may affect the operation

'/ of the new system—driveways, cross roads, | | Opposing cars passing other opposing cars, , |steep gradients, stopped vehicles, etc.?

'How will environmental conditions—rain, snow, ice, darkness—affect system opera- tions and how can they be overcome? What will be the costs and the benefits of a passing aid system?

Should information be given to drivers in visual, auditory, or tactile form? Preliminary answers to some of these ques- ons were obtained from experimental work

‘ith the PAS I system reported here. How- ver, most of the questions will be answered

uring PAS IT operations. Objectives of PAS I Study

The initial objectives of the first experi- 1ents with PAS I were as follows:

To determine whether drivers, even though their sight distance is restricted, will pass when they are informed that there are no Opposing vehicles within a specific distance.

q UBLIC ROADS ® Vol. 35, No. 3

e To ascertain that drivers will use speed information about the opposing vehicle to

aid them in passing.

¢ To obtain an indication of how long it takes drivers to adapt to a new system with one set of instructions.

e To determine whether clearance distances between passing and opposing vehicles at the end of passing maneuvers are adequate, based on use of 1,300-foot signal distances. The results of experiment 1 indicated that

drivers will make selective use of passing aid

information given to them by electronic means.

Additional planning of more sophisticated

passing aid systems is now well underway.

Description of PAS I

The first experiment was based on the use of a mockup of the Passing Aid System. The

mockup, called PAS I, was installed on the

access road to the Fairbank Highway Re- search Station and covered a distance of

approximately 0.7 mile. A simplified sketch of the PAS I test setup is shown in figure 1. The west bound direction road was used for the passing maneuver in which one vehicle, the passing car, was to overtake and pass another vehicle, the lead car, according to coded messages issued by the electronic passing aid system. The eastbound lane was used as the opposing lane in which an oncoming vehicle, the opposing car, approached the two west- bound vehicles in the east lane to provide a situation that required the driver of the passing vehicle to execute the passing maneuver in time to avoid a collision or to stay in his lane behind the lead car. Sight restrictors, installed along the roadway, obstructed the view of the road ahead and simulated the blind condition on 2-lane rural highways caused by hills and curves. Traffie detectors were spaced 44 feet apart in the lane used by opposing vehicles, and as the opposing vehicle moved

driver’s

over each detector, an intermittent audible signal The signal, which could be received by the passing

was. transmitted. intermittent car, was detectable at any point within 1,300 feet ahead of the opposing vehicle.

Four conditions could exist for the driver of the passing car: (1) No signal—the system was not operating, (2) a steady uninterrupted signal—the opposing lane was clear of traffic for at least 1,300 feet, (3) the beginning of an intermittent signal—there was a moving vehicle 1,300 feet ahead the lane, and (4) repetition of the intermittent 1,300

in opposing signal—a moving vehicle was within feet ahead in the opposing lane. The fre- quency/second of the increased with the speed of the vehicle. After the beginning of the signal, the number of intermittent signals and the

intermittent

signals

opposing

speed of the opposing vehicle indicated the clearance distance between the two vehicles.

Figure 2.—Diagrammatic representation of variables.

Test Subjects and Vehicles

Test subjects used in the two experiments were obtained from the student body of George Washington University, the Bureau of Public Roads staff and the general public. Information about the drivers is shown in table 1.

The passing and opposing vehicles driven by the test subjects were 1967 4-door sedans— Dodge, Valiant, and Plymouth—with the following specifications: automatic transmis- sion, power steering, power brakes, 6 cylinder, 225-cu. in. cylinder displacement, and 145- brake horsepower.

Table 2.—Minimum passing-sight-distance for design of 2-lane highways !

2 | Assumed Minimum Design speed | passing passing-sight- | speed distance

m.p.h, | m.p.h feet

30 | 30 800

40 } 40 1, 300

50 | 48 1, 700

60 55 2,000

70 | 60 2, 300

' Source: Blue Book, Geometric Design Rural Highways 1959, p. 211, : et

The lead car used in experiment 1 was a 1966 4-door Ford sedan with automatic trans- mission. The lead car in experiment 2 was a 1967 4-door Mercury sedan with automatic transmission, power steering, power brakes, 200-brake horsepower, 8 cylinders, and cyl- inder displacement of 289 cu. in. In general, the test drivers considered the power and performance of the vehicles they drove to be adequate.

The combination of the driver and the vehicle he drove for the first time presented significant variables that were significant in determining the acceptance of a passing aid system.

Description PAS I Study Variables

The variables considered in the preliminary studies are itemized in the following list, and where applicable, they are shown in figure 2:

Distance D =distance between passing and oppos- ing car. D.=signal range generated ahead of op- posing car, 1,300 feet. D;=D where passing may begin—any- where between the two flags. D,=D when passing maneuver begins. D.=D where passing maneuver ends. Speed Vi=speed of lead ear. V.=speed of car following lead car prior to passing maneuver. V3=speed of opposing car. Time T,=time required to pass. T;=time for test car to reach juxtaposi- tion with opposing car after having completed passing maneuver.

Time, in addition to distance and specd, was observed in the hope that it would serve as a check—distance=speed X time—and be useful for the period covered by the passing maneuver when acceleration and _ speeds change significantly.

Two types of test runs were used in the experiments—radio and control. In the radio, or PAS I, test runs, the electronic passing aid system was used by the driver of the passing car to overtake and pass the lead car. The control test runs were made without the use of the passing aid system and were included in the experiments to provide a basis of com- parison in analyzing the effectiveness of PAS I. The 2-lane test roadway had a design speed of 70 m.p.h., a posted speed limit of 30 m.p.h., and two long, 3-degree curves with a tangent between them. The posted speed limit was not in effect for the test runs.

Discussion of Variables

Preliminary test data, collected prior to experiments 1 and 2, indicated that several variables would have to be controlled.

The first variable was sight distance. Sight distances were so large it would have been difficult to determine whether there was any difference in the frequency of passing maneu- vers between the control and simulated PAS I test runs. To decrease passing opportunity and more closely simulate driving conditions that would exist on a rural mountainous road, temporary panels were installed (see fig. 1) to restrict the sight distance. To insure com- parable passing opportunities for the test and control situations, the sharpest curve, near the midpoint of the test road, was used. The part of the curve between stations 150 and 125 was selected as the section of roadway

PASSING AID SYSTEM I

Test Subject:

EXPERIMENT NO, 1

Observer:

where passing maneuvers could begin. He sight distances were 620-1,300 feet, bi temporary panels reduced them to 400-5 feet. The sight distance was based on ft ability of the driver in the right lane to s any part of a vehicle in the opposing la The second variable to be controlled wa the frequency at which an opposing vehiel was encountered in the passing area. For th control phase of preliminary test runs, driver were instructed to drive the way they nor mally drive, but the frequency of passin maneuvers seemed abnormally high. Tes drivers confirmed this by volunteering th information that normally, on the open high way, they would not pass if the sight distane were comparable. 4 Three possible reasons were considered fe the incongruity between drivers’ statement and actions. The first was that the test roa was always cleared of other traffic so the passing manuevers could be based solely o the position and speed of the opposing te vehicle. This was a definite requirement fe study of PAS I and consequently, to perm a comparison, it was also a requirement f¢ the control phase. Because test drivers kne there would be only one vehicle in the opposir lane and that the driver of the opposing ¢& would be aware of the passing maneuver, the were more willing to pass. They apparent believed that they were not fully responsib for the passing maneuver and its possible co sequences, as they are on the open road. — Another possible reason for the discrepant between the statements and actions of ft) drivers was that they were speaking in gener terms based on normal operating speeds. F example, table 2 gives minimum passing sig distances of 800 and 1,300 feet at speeds

FIELD DAT

Date: Time Begin:

Station Number

Time-0,01 min,

Passing | Opposing car (Units=passing car. Tens=opposing car, Twenties=passed car) ks zero Run car Start t : No. |Series |(m.p.h.) | m.p.h.| Station Lyi Rati eo Se M93 ive Remarl 51 [149 us | 1os| roe | 11 132 PASS h 102 Sift

= 30 | Pass ree es a

—

bate eee Patines ea ia

PF C Q

| sees

= a

pe] * | 4s | he | =| [oe

~ = fe} ~

ss fo | = | 30 [4s

Figure 3.—Sample data sheet, experiment No. 1.

, a oe Ue _—

August 1968 ® PUBLIC RO

0 and 40 m.p.h. respectively. If these speeds ere considered normal, it would have been nsafe to pass after installation of the tem- orary sight-restrictor panels, because the aximum sight distance available in the des- nated passing area was less than 550 feet. t 20 m.p.h., however, the minimum passing ight distance would have been approximately e same as the sight distance available, and e test subjects should have been willing to ass with or without the use of PAS I. The referred approach was to study conditions which passing maneuvers normally were ot feasible, and it was decided to eliminate st runs based on a lead car speed of 20 m.p.h. A third possible reason for the incongruity vetween drivers’ statements and actions was at the opposing vehicle and the passing ‘ehicle seldom were near the passing area }limultaneously, and drivers may have realized hat it was usually safe to pass the lead car. Any of these possibilities or combinations f them, could have accounted for the high vassing frequency in the control phase of the reliminary tests. To eliminate the first |vossibility, decreased driver responsibility, e following driving instructions were issued “lo the test subjects; these instructions re- laced the game aura of the experiment with ne of responsibility: || Entering the car.—‘Please fasten your safety “helt. The purpose of this research study is to “\Inalyze how you drive so we may develop ids to other drivers.”’

Test runs, control phase.—‘‘Please start the iar. Drive as you normally would on this -lane highway. Follow the car ahead. There 4 vill be traffic coming toward you in the | |Pposing lane. If in your judgment you would ‘ vormally pass the car ahead, you are free to do 0 by beginning your passing maneuver some- vhere between the two red flags along the left ide of the road. If you do not consider it safe 0 pass, continue to follow the car ahead. a Drive safely. Take no chances. Drive in a

aanner similar to the way you drive on the q \ighway. Any questions?”

| Test runs, passing aid phase.-—‘‘When you i \ear a continuous tone, from your radio ~ eceiver, the opposing lane is clear of moving

ASS ch ° ~~ traffic for at least 144 mile. When a vehicle

“3 moving toward you in the opposing lane — loser than 14 mile, you will hear beeps on — he radio. If you desire to pass, you may use — |he radio signals to aid you in deciding whether _ ir not to pass.

— “As before, if you do choose to pass the car — head, the passing maneuver should start in —\he_ area between the red flags. Any _ {uestions?”’

—| The second possibility was eliminated by _ lisearding the 20-m.p.h. test runs, mentioned _\arlier, and the third by increasing, for each Vest subject, the percentage of runs in which here was no opportunity to pass in the ~ yassing area. For the no-passing situation to ~»eceur in the designated passing area, it was ~ tecessary to specify not only the lead car and ~ »pposing vehicle speeds, but also the stations “irom which the vehicles would begin each

jest run.

: JUBLIC ROADS © Vol. 35, No. 3

START

STATION POSITION, m

t t= 0 e

FINISH CONTACT . €

ELAPSED TIME FROM t,, HUNDREDTHS OF MINUTES (0.0i)

Figure 4.—Vehicle positions at which station numbers and elapsed time was recorded.

Experiments and Precedure

Data were collected in two series of tests— experiment 1 and experiment 2. In both experiments the test subjects were used in pairs. For a test run, one subject would operate the passing vehicle and the other subject the opposing car. For the next test run, the drivers exchanged assignments so that each driver was used coming and going in each pair of runs.

Experiment 1

Data for experiment 1 were recorded on the form shown in figure 3. The first five vertical colums at the left contain the previously dis- cussed control variables. The run number

indicates the individual trips on the test road during which a passing maneuver could occur. The column originally indicated the trip sequence for both drivers, but midway through experiment 1, this arrangement was deter- mined to be undesirable, as one driver of each pair of drivers would operate the opposing vehicle in one run, then operate the passing vehicle in the next run under identical test conditions. Consequently, he could recall the starting position of the opposing vehicle, its speed, the clearance distance available for passing, or any one of these factors, to formu- late a predetermined pass or no-pass decision. In the field it was decided to eliminate the

Speed combination

Ne. PEA eee, om

Table 3.—Data summary for experiment 1, test subjects Nos. 12-17 !

Begin run

Lead car

| Opposing

car

Lead car

Opposing |

car

Opposing lane clear for more than 1,300 ft. at Ist flag

Pass/No-pass frequency

mS (abet be Seated Ta’ Le take ye een. _ cP We +? . F we .

Passing percentage

Control

PASI

Control

Pass

m.p.h.

180 180

Station No.

180

180 180 180

1 Test subjects are listed in table 1.

Speed combination

Station No.

27 50 88

Number

No-pass

Pass

No-pass

Number

Test

Percent

Total

Percent

PASI

Test

Percent

4 Total ;

fi Percent

Table 4.—Data summary for experiment 2, test subjects Nos. 18-23 !

Begin run

Lead car

| Opposing car

Lead car

Opposing car

Opposing lane clear for more than 1,300 ft. at Ist flag

m.p.h. 30

m.p.h. 15

Station No. 180 180

180 180

1 Test subjects are listed in table 1.

Speed combination

Station No. 517

ai 88

Pass/No-pass frequency

Passing percentage

Control

PAS I

Control

Pass

Number 2

No-pass

Number 4

Pass

Number 4

No-pass

Number 9

Test

Percent

Total

Percent

Table 5.—Data summary for experiments 1 and 2 combined, test subjects Nos. 12-17 and 18-23 !

Begin run

1 Tests subjects are listed in table 1.

Pass/No-pass frequency

Passing percentage

PASI

Test

Percent 66

Total

Percent

Opposing lane clear for more than 1,300 ft. at y 7 Ist flag Control PASI Control PAS I Lead car | Opposing | Lead car | Opposing car car Pass No-pass Pass No-pass Test Total Test Total m.p.h. m.p.h. | Station No. | Station No. Number | Number | Number | Number | Percent | Percent | Percent | Percen 30 15 { 180 27, Mies 2. 500 ees en) Cee ene. Oe ire + 6 5 2 40:45 spheres 71 -- ‘ | 180 88 pe ke | Sie eee see iN. 4 9 5 9 31” Av eeeat 36 ae | ‘Total: passing fee se eee l= Percents si" a2! pe | Pe nee ee ae SD Ca eieeee Sree 48 : 180 27 ViO8. 25. Sst nn ees oo ee SRS eg 2S 3 us 17 4 30 30 a Ba. A No kee eas = Reine eH i 5 25 || i ae eee | ‘Total pussitre® ons eee percent} eine Cac Ne eee 2 eee 20 ol ae “68 : Be ad 180 27 BME 2 Se ee eS a 0 6 0 6 0 45 a rt Bab | Nose eee ae CaP es & aa 1 13 1 1 7 | ae ieee ae ‘Dotal Passive) oe Beach te POTCRNE 22} ee 2a Sos eas NS Be ee eee ee (at Se a 45 45 { 3 27 OOK 226s ae Ta Bae on, bs eee Nw Oe 3 9 8 20 aa | ee 50 in

August 1968 ® PUBLIC RO