JOHN R. BROWN, Chief Judge:
We are asked to review the lower court’s determination that appellee’s (Tamiami) policy of refusing to consider applications of individuals between the ages of 40 and 65 for initial employment as intercity bus drivers is a bona fide occupational qualification (BFOQ) reasonably necessary to the normal operation of its business. Typically statutory, the words “reasonably necessary to the normal operation of its business,” are not normally of the variety that suggest hand-wringing, earth-shaking, heart-rendering decisions of great moment. Words can be deceiving though, and we readily perceive and admit that our task in this controversy is not a simple one. Inextricably involved here, on the one hand, is perpetuation of the stated legislative purpose to promote employment of older persons based on their ability rather than age, undeniably a laudable one, which must be somehow balanced against the unquestionably sincere claims of the nationwide busing industry which, as expressed by some of its most eminent spokesmen, has amassed a safety record unsurpassed by any other transportation industry so that the public interest — here a term directly translatable into the safety of millions of intercity bus passengers per year — demands that the non-hiring of intercity bus drivers over 40 years of age be sanctioned as a necessary exception. After reviewing the record at length we are convinced that the conclusions of the District Judge were not clearly erroneous. F.R.Civ.P. 52(a). We affirm.
The Proceedings Below
This action was brought by the Secretary of Labor, United States Department of Labor under the provisions of Section 41 of [227]*227the Age Discrimination in Employment Act of 1967, 29 U.S.C.A. § 621, et seq., seeking to permanently enjoin Tamiami Trail Tours, Inc. from allegedly denying employment to individuals within the age group protected by the Act2 and from withholding payment for unpaid wages allegedly due eight individuals (the complaining witnesses herein) because of discrimination against them. In its answer Tamiami admitted that it refused to consider two of the eight complaining witnesses solely on account of their age.3 With respect to the other six complaining witnesses the defendant contended that it would not have employed them in any event because of reasons other than age.4 It asserted, however, that such refusal was done pursuant to the BFOQ exemptions provided in § 4(f)(1) of the Age Discrimination in Employment Act of 1967.5
Thus, the issues for the liability portion of the trial,6 as reflected by a pre-trial stipulation entered into by the parties, became (i) whether Tamiami’s refusal to hire each of the six complaining witnesses listed in the complaint to which their answer indicated a denial was, in actuality, because of the age of each such person, (ii) respecting the two complaining witnesses to which Tamiami’s answer admitted denial of employment based upon age, whether age with respect to the position of bus driver is a bona fide occupational qualification (BFOQ) reasonably necessary to the normal operation of defendant’s business within the meaning of § 4(f)(1) of the Age Discrimination in Employment Act, and (iii) if so, whether Tamiami’s violations were such that injunction would be warranted.
The trial court found that with respect to complaining witnesses Wons and Poole (see note 3, supra) a prima facie case of age discrimination had been made. This, the Court recognized, served to shift the burden of proof to the defendant to justify its hiring policy. From there the trial judge reasoned that whether Tamiami’s reliance on § 4(f)(1) of the Act could be a valid defense depended essentially on questions of Tamiami’s safety obligations, safe transportation of passengers being the essence of the motor carrier’s operations as this Court held in Diaz v. Pan American World Airways, Inc., 5 Cir., 1971, 442 F.2d 385, cert. denied, 404 U.S. 950, 92 S.Ct. 275, 30 L.Ed.2d 267. Thus, in the District Court’s view, Tamiami could sustain its burden of justifying its application of the general rule (i) by showing that it had a factual basis for believing that otherwise its business operations (safety obligations) would be undermined 7 and (ii) by demonstrating that dealing with each applicant over 40 years of age on an individual basis by considering his particular functional ability to perform [228]*228safely the duties of a driver notwithstanding his age, would be impractical.8
In support of its defense, Tamiami produced numerous witnesses who testified that, in their opinion, age was a reasonable and necessary employment qualification for the position of intercity bus driver. Representatives of other bus companies testified that their practice (see note 2, supra) was similar to that of Tamiami and was grounded upon reasons of safety. This testimony set out with painstaking clarity the great lengths to which bus companies go to promote safety. Many of Tamiami’s witnesses emphasized its strong contention that the rigors of extra board (the meaning of this term to be discussed in detail infra) are such as to necessitate the imposition of an age limitation. So testifying were bus company executives as well as bus drivers themselves, both junior and senior in terms of past service.
Introduced also was conflicting expert testimony regarding the effects of the aging process upon the ability to safely perform the duties of bus driver and upon the reliability of an annual or recurringly scheduled full physical examination used as a screening process to detect those physical and sensory changes common to all persons which result from the aging process.
In his findings of fact and conclusions of law, the trial judge characterized the dispute as not being such a case of “arbitrary” age discrimination as contemplated under the Act. The court concluded that the extraordinarily high degree of care required of bus companies in hiring drivers coupled with the fact that the seniority system imposes roughly a 10-year period on the extra board, justified the industry’s utilization of an age limitation as the best available tool for screening out unsuitable driver applicants. The Court concluded from the evidence that functional age, as distinguished from the chronological age, of a driver applicant over 40 cannot be determined with sufficient reliability to meet the special safety obligations of motor carriers of passengers. Finding (i) that the defendant had demonstrated a factual basis for its belief that all or substantially all men over 40 would be unable to perform safely and efficiently the duties of the job of intercity bus driver and (ii) that the weight of the evidence introduced supported the view that recurring physical examinations and other testing methods are unreliable in detecting certain of the mental and psychological changes inherent with aging such that individual consideration of applicants over 40 would be highly impractical, i. e. untrustworthy, the court entered its judgment for Tamiami. The court further found that its ruling upholding the 40 year age limitation with regard to the two complaining witnesses for whom the defendant had admitted age was the sole reason for non-hiring 9 made it unnecessary to reach the issues relative to the three remaining complaining witnesses.10
The Act
President Johnson, in his Older American Message of January 23,1967, rec[229]*229ommended the Age Discrimination in Employment Act of 1967, which was transmitted to the Congress by the Secretary of Labor in February of that year. The President’s message, in the section on job opportunities, stated—
Hundreds of thousands, not yet old, not yet voluntarily retired, find themselves jobless because of arbitrary age discrimination. Despite our present low rate of unemployment, there has been a persistent average of 850,000 people age 45 and over who are unemployed. Today more than % of the billion dollars in unemployment insurance is paid each year to workers who are 45 and over. They comprise 27% of all the unemployed * * * In economic terms, this is a serious — and senseless — loss to a nation on the move. But the greater loss is the cruel sacrifice in happiness and well-being, which joblessness imposes on these citizens and their families. Opportunity must be opened to the many Americans over 45 who are qualified and willing to work. We must end arbitrary age limits on hiring. [113 Cong.Rec. pp. 34743-34744, Dec. 4, 1967.]
Recognizing the seriousness of the problem,11 Congress enacted the Age Discrimination in Employment Act in 1967 for the express purpose of promoting “employment of older persons based on their ability rather than age” and prohibiting “arbitrary age discrimination.” The Act makes it unlawful for employers, employment agencies, and labor organizations to discriminate on the basis of age,12 against persons between the ages of 40 and 65.13
Section 4(f)(1) provides the BFOQ statutory defense to actions brought for violation of the Act under certain restrictive circumstances.14 Pursuant to its usual practice, the Department of Labor promulgated an Interpretative Bulletin indicating the construction of the Act which it believed correct to be used as an aid for employers and employees to comply with its provisions. With regard to BFOQ, this Bul[230]*230letin states that “whether occupational qualification will be deemed to be ‘bonafide’ and ‘reasonably necessary to the normal operation of the particular business’ will be determined on the basis of all the pertinent facts surrounding each particular situation.” The Bulletin further anticipates “that this concept of a bonafide occupational qualification will have limited scope and application” and reiterating, states that “further, as this is an exception it must be construed narrowly, and the burden of proof in establishing that it applies is the responsibility of the employer which relies upon it.” The Bulletin continues on to list as an illustration of a possible bonafide occupational qualification those situations where “federal statutory and regulatory requirements provide compulsory retirement, without reference to the individual’s actual physical condition at the terminal age, when such conditions are clearly imposed for the safety and convenience of the public.” The Bulletin cites as an example the situation where federal aviation agency regulations do not permit airline pilots to engage in carrier operations as pilots after they reach age 60. (29 C.F.R. § 860.102).
In the section of the Interpretative Bulletin entitled “differentiations based on reasonable factors other than age” it is again stated that “the reasonableness of differentiation will be determined on an individual, case by case basis, not on the basis of any general or class concept, with unusual working conditions given weight according to their individual merit.” The Bulletin states that “a differentiation based on a physical examination, but not one based on age, may be recognized as reasonable in certain job situations which necessitate stringent physical requirements due to inherent occupational factors such as the safety of the individual employees or of other persons in their charge, or those occupations which by nature are particularly hazardous.” It is then stated that “a claim for a differentiation will not be permitted on the basis of an employer’s assumption that every employee over a certain age in a particular type of job usually becomes physically unable to perform the duties of that job.”
That these interpretations by the agency charged with the enforcement of the statute are entitled to considerable weight is well settled. Udall v. Tallman, 1965, 380 U.S. 1, 85 S.Ct. 792, 13 L.Ed.2d 616; Griggs v. Duke Power Co., 1971, 401 U.S. 424, 91 S.Ct. 849, 28 L.Ed.2d 158.
The Industry
Tamiami Trail Tours, Inc. is an intrastate and interstate motor common carrier of passengers and baggage operating under certificates of public convenience and necessity issued by the Interstate Commerce Commission and the Florida Public Service Commission. Founded in 1924 and headquartered in Miami, Florida, Tamiami is the oldest bus company in Florida and today provides bus transportation services for the general public in all of Florida, South and Central Georgia and a portion of Alabama. Tamiami participates as one of the larger members of the national trailways system which is a non-profit association of intercity bus companies operating under the common trade name of “Trailways”. As a regulated common carrier, Tamiami daily conducts in-. numerable scheduled runs, sight-seeing tours and other special operations for the pleasure and convenience of the traveling public. The sight-seeing tour and charter feature of Tamiami’s operation is designed to satisfy the travel needs of a predesignated group of passengers making a specific journey for some mutual purpose and then returning upon completion to the point of origin. Such operations may vary from short cross-city travel to a three or four week transcontinental charter.
With respect to bus drivers, Tamiami follows an industry-wide policy of conducting operations under a seniority system which is embodied in its labor contracts. Under the system there are two general classifications of drivers — those who perform “regular runs” and those who perform “extra board.” Extra board drivers consist of operators who have insufficient seniority to [231]*231successfully attain a regularly scheduled run.
The Extra Board
The nature of Tamiami’s business, and for that matter, that of other major bus companies also, necessitates that they operate with approximately 25 to 30% more drivers than would be required to drive the regularly scheduled runs. Accordingly, bus drivers bid for preferred runs on the basis of seniority. Within Tamiami, the existing seniority system relegates a driver to approximately 7 to 12 years of extra board driving before he has sufficient seniority to bid for and obtain a regular route. In such status, extra board operators are called upon to take up the slack in normal operations and to fill in for regular drivers in the event of vacation, sick leave and the addition of extra buses to regularly scheduled runs where needed. Additionally, these drivers are responsible for Tamiami’s charter services which constitute a significant portion of its total operations.
The term “extra board” actually refers to a schedule where the drivers are listed on a first in, first out basis. As a driver completes an assignment and returns to his home terminal, he is then placed at the bottom of the extra board list. As additional assignments (over and above the regularly scheduled runs) are to be made by Tamiami, the extra board man whose name is at the top of the list is designated as the driver. A man on the extra board has no flexibility or freedom of choice concerning his assignments. He must take whatever assignment comes up and complete it, whether it be a two week charter to another state or a one-day sight-seeing tour. With each assignment, the next driver on the extra board moves into the “first out” slot, and so on down the list.15
There can be little doubt that, since extra board drivers are on call 24 hours per day, 7 days a week, and must be prepared to go anywhere in the continental United States at any time on short notice, the work is demanding and physically exhausting. Although there is a provision under which a driver must be given two hours notice prior to departure time for short trips, 8 to 24 hours prior notice for longer trips,16 tight scheduling and unforeseen demands frequently greatly diminish a driver’s ability to predict the time of his next assignment. As such, it can hardly be disputed that the unpredictability, coupled with the strains and rigors of extra board driving would almost certainly render an extra board driver’s opportunity for a normal home or family life more difficult than that of a driver with a seniority-attained scheduled run.
Governmental Safety Regulation
Motor bus carriers, along with all common carriers, are charged both under considerations of public policy and by the operation of law with exercising an extraordinarily high standard of care for the safety of their passengers. Pursuant to the Interstate Commerce Act17 as well as other [232]*232laws,18 the Bureau of Motor Carriers Safety has issued detailed regulations affecting drivers 19 among other aspects of motor car-, rier passenger operations (49 C.F.R. Parts 390-396). These regulations prescribe minimum qualifications for the drivers of motor vehicles and minimum standards governing the forms of employment applications, written and road tests to be given, and physical requirements applicable to drivers (49 C.F.R. Part 391). They further impose the rules under which motor vehicles shall be driven (49 C.F.R. Part 392). The hours of service permitted drivers are also set out in detail (49 C.F.R. Part 395) (see note 16, supra). That these regulations are minimum standards only is expressly provided—
Nothing in parts 390-397 of this subchapter prohibits a motor carrier from requiring and enforcing more stringent rules and regulations relating to safety of operation (49 C.F.R. Part 392.1(b)).
The Department of Transportation has instituted programs for surveillance to insure compliance with these regulations by drivers and carriers. The fifth annual report of the Department of Transportation for fiscal year 1971 contained a section on motor carrier safety which described such activities of the Government.20
Tamiami’s Practices
Tamiami, certainly not alone in the busing industry, has paid careful heed to the rigorous federal and state regulations governing such operations. Commendably, their operations for selection of drivers are exercised with the utmost care for insuring the safety of the traveling public. The hiring process at Tamiami begins with a written application form designed to eliminate those applicants who are unqualified.21 Those applicants who are not eliminated initially by the written application, must [233]*233next be interviewed and approved by Tamiami’s personnel officer or his designated delegate. In addition an applicant must receive a favorable report from a retail credit company. All applicants are subject to thorough physical examination by a qualified physician.
Once hired, the drivers are given recurring physical examinations on an annual basis (although the Department of Transportation requires physical examinations only once every two years). The physical examination given to all applicants and incumbent drivers is extremely thorough and rigorous. Indicative of the examinations thoroughness is that under vision, the physician must check peripheral vision, distance vision and color vision. The Department of Transportation issues instructions to examining physicians advising them that “the examination should be made carefully * * * ” and provides each with detailed minimal instructions.22 Further guidance prescribes that “History of certain defects may * * * indicate the need for making certain laboratory tests or a further, and more stringent, examination.”22 The Department of Transportation cautions the examining physician to keep in mind “the rigorous physical demands and mental emotional responsibilities placed on a driver of a commercial motor vehicle,” pointing out that “in the interest of public safety, the examining physician is required to certify that the driver does not have any physical, mental or organic defects of such a nature as to effect the driver’s ability to operate safely a commercial motor vehicle.”22 Additionally, Tamiami interviews each examining physician to make absolutely certain that he is familiar with the requisite physical standards.
Tamiami’s training program for new drivers is conducted by their own personnel and lasts approximately six weeks, the bulk of which time is spent in operating the bus under all conditions, including night driving, under the supervision of either an instructor or one of the regular bus drivers who is required to fill out a form evaluating the student’s performance. Upon completion of the training course, the student driver is given a comprehensive road test, which requires the examiner to grade the student on 120 different driving skills (parking, steering, stopping, slowing, passing, turning, etc.), using a form prepared by the Department of Transportation. The examiner also grades the student on his alertness and attentiveness, willingness to take instructions and suggestions, degree of nervousness and apprehensiveness, whether the applicant is easily angered, and physical stamina. Tamiami further continuously evaluates its drivers in terms of their job performance, safety record, attitude and physical fitness and, on the basis of these evaluations, may require particular drivers to report for more frequent physical examinations or to undergo periodic retraining.
Throughout its history of operation, Tamiami has compiled an enviable record of safe, accident-free driving mileage and has been recognized within the motor bus industry by many highly coveted safety awards.
The Question
With this as background, the question presented is whether the conclusion of the District Judge that age is a bona fide occupational qualification reasonably necessary to the safe operation of Tamiami Trail Tours, Inc. is clearly erroneous. F.R.Civ.P. 52(a); Zenith Radio Corp. v. Hazeltine Research, 1969, 395 U.S. 100, 89 S.Ct. 1562, 23 L.Ed.2d 129; Chalk v. Beto, 5 Cir., 1970, 429 F.2d 225. The Government specifically challenges this conclusion on the grounds that (1) the evidence showed conclusory assertions — not the “factual basis” we required in Weeks to establish the BFOQ, and (2) that there was insufficient evidence to show that it would be highly impractical to evaluate older driver-applicants on an individual basis. Tamiami, the Government contends, should have been made to compare the accident records of its extra board drivers over 40 with the accident rates of its under 40 extra board drivers in order to [234]*234carry its burden. The Government suggests that the conclusions of the District Judge generally represent precisely the stereotypical thinking that the ADEA was designed to prevent.
The Safety Factor And The Weeks-Diaz Test
We are not the first Circuit to consider this question. In Hodgson v. Greyhound Lines, Inc., 7 Cir., 1974, 499 F.2d 859, cert. denied sub nom., Brennan v. Greyhound Lines, Inc., 1975, 419 U.S. 1122, 95 S.Ct. 805, 42 L.Ed.2d 822, the Seventh Circuit reviewed the even more exclusive hiring policy of Greyhound precluding the employment of intercity bus drivers 35 years or older. The District Judge had determined that Greyhound failed to carry its burden to demonstrate a factual basis for its belief that its policy was reasonably necessary for the safe operation of its business.23 Although the District Judge relied primarily on our Weeks case, the Court of Appeals chose to shift the focus for determining the standards to prove up the defense. In view of the overriding public safety factor not present in Weeks, the Seventh Circuit, relying on Diaz,24 required Greyhound to demonstrate that the essence of its business would be endangered by hiring drivers who were over 40.25
The Seventh Circuit in Greyhound concluded that
Greyhound need only demonstrate however a minimal increase in risk of harm for it is enough to show that elimination of the hiring policy might jeopardize the life of one more person than might otherwise occur under the present hiring practice.
499 F.2d at 863.
In reassessing all of the evidence under this revised standard, the Court concluded that Greyhound by demonstrating the rigors of the extra board and the degenerative effects of age upon the human body, plus the statistical evidence showing that the profile of the safest Greyhound driver shows him to be approximately 50 to 56 years of age with 16 to 20 years experience (which he could not acquire if hired after 40) adequately demonstrated that Greyhound’s position that elimination of its maximum hiring age would increase the potential risk of harm to its passengers was grounded on an adequate factual basis. The Court therefore held the findings to the contrary to be clearly erroneous.
The Greyhound court distinguished Weeks on the ground that “the Fifth Circuit was not confronted with a situation where the lives of numerous persons are completely dependent on the capabilities of the job applicant.” 499 F.2d at 861-62. The question raised, therefore, is whether the Weeks requisite of the BFOQ test [235]*235should be dropped or modified when the safety factor is present. Though we agree with the Greyhound court that the safety of third parties is a factor which cannot be ignored, we believe that this safety factor is already appropriately highlighted within the current framework of the Weeks-Diaz test.
In Weeks we held that the employer’s refusal to hire women for a job that required occasional strenuous activity violated the Title VII prohibition of employment discrimination. In rejecting the employer’s assertion of the BFOQ defense, we set out the test for qualification for that defense:
an employer has the burden of proving that he had reasonable cause to believe, that is, a factual basis for believing, that all or substantially all women would be unable to perform safely and efficiently the duties of the job involved.
408 F.2d at 235. However, even when the employer cannot carry this burden, if it demonstrates “that it is impossible or highly impractical to deal with women on an individualized basis, it may apply a reasonable general rule.” 408 F.2d at 235 n. 5. (See note 8, supra.) One method by which the employer can carry this burden is to establish that some members of the discriminated-against class possess a trait precluding safe and efficient job performance that cannot be ascertained by means other than knowledge of the applicant’s membership in the class.26
The second element of the employer’s burden under the BFOQ defense was set out in Diaz, in which we invalidated the employer’s policy of refusing to hire males for jobs as airline cabin attendants. The statutory requirement that the BFOQ be “reasonably necessary” to the operation of the business was construed to limit qualification for the defense to those cases in which “the essence of the business operation would be undermined by not hiring members of one sex exclusively.” 442 F.2d at 388. We then noted that “[t]he primary function of an airline is to transport passengers safely from one point to another.” Id. See note 29, infra. We held that the employer could not carry its burden under the BFOQ defense because, although most males may not be able to adequately perform the nonmechanical functions of a cabin attendant, this would not undermine the safe transportation of passengers.27
It is in the Diaz element of the BFOQ defense that the third-party safety [236]*236factor comes into play. Diaz mandates that the job qualifications which the employer invokes to justify his discrimination must be reasonably neeessary to the essence of his business — here, the safe transportation of bus passengers from one point to another. The greater the safety factor, measured by the likelihood of harm and the probable severity of that harm in case of an accident, the more stringent may be the job qualifications designed to insure safe driving. Thus, it is the Diaz element rather than the Weeks element of the BFOQ defense which adjusts to the safety factor.
It is important to understand that once an employer’s job qualifications have passed muster under Diaz, the Weeks element of the BFOQ test does not mandate hiring applicants who do not meet these perhaps stringent qualifications. On the contrary, if all or substantially all members of a class do not qualify, or if there is no practical way reliably to differentiate the qualified from the unqualified applicants in that class, it is precisely then that Weeks permits otherwise proscribed class discrimination as a BFOQ. The Weeks test does not mandate either the hiring of unqualified applicants or applicants whose qualifications cannot be reliably determined, whether or not they are black or female or over 40 years old.28
No one has suggested that Tamiami’s stringent job qualifications (discussed above at p. 233) for the position of bus driver are either unrelated29 to the essence of the business or unreasonable30 in light of the safety risk. Tamiami has unquestionably satisfied the Diaz prong of the BFOQ test.
Having overcome this threshold obstacle which tripped Pan Am in Diaz, Tamiami faces the second, double-faceted prong of the BFOQ test set out in Weeks : whether it had reasonable cause, that is, a factual basis, for believing that all or substantially all persons over 40 would be unable to perform safely and efficiently the duties of the job involved, or whether it is impossible or impractical to deal with persons over 40 on an individualized basis. See note 8, supra.
Tamiami did not attempt to establish a factual basis for believing that substantially all job applicants over 40 years of age would be unable to drive buses safely. It is thus relegated to the “footnote 5” burden in [237]*237Weeks of demonstrating that the passenger-endangering characteristics of over-40year-old job applicants cannot practically be ascertained by some hiring test other than automatic exclusion on the basis of age.
Trial Court’s Findings Supported
Much of the evidence submitted by Tamiami on the BFOQ issue, and relied upon by the District Court, is either irrelevant to the Weeks (footnote 5) standard or insufficient to carry Tamiami’s burden.
(1) For about his first ten years of employment, the bus driver works on the “extra board,” which often requires driving long routes in the early morning hours with little advance notice. Tamiami contends that older drivers should not be subjected to such pressures. Women now have the same right “[m]en have always had ... to determine whether the incremental increase in remuneration for strenuous, dangerous, obnoxious, boring or unromantie tasks is worth the candle.” Weeks, 408 F.2d at 236. Similarly, over-forty-year-old workers should have the same opportunity as do their younger counterparts to take on the pressures of extra board driving, assuming that this does not increase the risk to passengers. The above argument therefore cannot satisfy Tamiami’s BFOQ burden under the footnote 5 alternative of Weeks unless it establishes that there is no hiring test that would distinguish between those older drivers who can drive safely despite the pressures and those who cannot.
(2) Because of the established seniority system, Tamiami argues that older bus drivers can compensate for the decline in physical attributes that is part of the aging process by choosing day shifts and short routes. If older applicants were hired, however, they could not chose the easier shifts because they would not have seniority. One flaw in this argument is that the seniority system is not necessarily beyond challenge under the ADEA. Moreover, the fact that taking on the lighter load is voluntary must reflect Tamiami's view that safety factors do not mandate that older drivers not be allowed to work the extra board.
(3) Tamiami presented statistical evidence indicating that there is a correlation between age and accident frequency. This is an insufficient showing because an examination may reveal whether a job applicant possesses any of the passenger-endangering physical liabilities that many older people have. In other words, the correlation is to physical deficiencies, not to age per se.
Tamiami, however, did present proof that, if accepted, would satisfy its BFOQ burden under Weeks (footnote 5). Tamiami introduced testimony by both medical and transportation experts. In particular the testimony of Dr. Harold Brandaleone, an eminent medical expert in the field of transportation and motor vehicle accidents, was heavily weighted by the Court. Dr. Brandaleone testified that while chronological age could not be isolated as a factor automatically indicating that an individual could not adjust to the rigors of the extra board schedule, medical science could not accurately separate chronological from functional or physiological age. According to him, (1) certain physiological and psychological changes that accompany the aging process decrease the person’s ability to drive safely and (2) even the most refined examinations cannot detect all of these changes. In his opinion 40 years of age was by no means an arbitrary cutoff to enable bus companies to screen out such impairments.31
In addition to this medical testimony, Tamiami called Dr. Ernest G. Cox, former Safety Director for the Bureau of Motor Carriers in the Interstate Commerce Commission and later Deputy Director of the Bureau of Motor Carrier Safety in the Department of Transportation to explain the strains on intercity drivers and the importance of the driver’s relationship to traffic safety. Dr. Cox expressed the unequivocal opinion that the maximum age qualification was essential to the normal and safe operation of intercity bus lines.
[238]*238Lastly, Tamiami called five of its current drivers — both senior and extra board juniors — to describe their experiences with the driving conditions of the extra board and regular runs. It was clear from their testimony that acquiring enough seniority to get off the extra board was a primary goal of every driver. Despite their experience, the regular run senior drivers emphatically believed that they could not return to the extra board and still maintain the safety of their passengers.
In contrast to Dr. Brandaleone’s medical testimony, the Government relied primarily on the rebuttal testimony of Dr. Abraham J. Mirkin, an expert on automotive accidents, who dismissed any relationship between age and one’s ability to drive a vehicle safely, and Kenneth Pierson, Dr. Cox’ successor as Deputy Director of the Bureau of Motor Carrier Safety of DOT, who believed that the physical examination, training program and road test that all new drivers are subjected to were sufficient to screen out those applicants of any age who would not be qualified to be safe bus drivers.
The key question is whether a sufficient factual basis for Tamiami’s case has been demonstrated by the catalogued strains and pressures of the extra board on the drivers who must serve it, the evidence regarding the inevitable physiological degeneration that accompanies age, and in particular the testimony of Dr. Brandaleone — which the District Judge was entitled to credit fully — that the available tests cannot distinguish those drivers not yet affected by the more crucial age-related accident-causing impairments like loss of stamina, etc. Whatever might have been our finding were we deciding this issue de novo, the District Court found that examinations cannot detect the relevant physiological and psychological changes “with sufficient reliability to meet the special safety obligations of motor carriers of passengers.” Hodgson v. Tamiami Trail Tours, Inc., S.D.Fla., 1972, 4 E.P.D. 17795, at 6051. This is a finding of fact and, therefore, may not be set aside unless “clearly erroneous.” Fed.R.Civ.P. 52(a). Because of the testimony of Dr. Brandaleone, we do not believe the District Court’s finding can be so categorized.32
Refrain
This interpretation of Weeks-Diaz clarifies the significance of safety to passengers and members of the public. We emphasize safety in busing just as we would in a variety of other industrial areas where safety to fellow employees is of such humane importance that the employer must be afforded substantial discretion in selecting specific standards which, if they err at all, should err on the side of preservation of life and limb. The employer must of course show a reasonable basis for its assessment of risk of injury/death. But it cannot be expected to establish this to a certainty, for certainty would require running the risk until a tragic accident would prove that the judgment was sound. Priceless as is a single life in our concept of the value of human life and our undoubted unwillingness ever to approve a practice which might kill one but not, say, twenty, we think the safety factor should be evaluated in terms of the possibility or likelihood of injury/death.
The ADEA was designed to eliminate unjustifiable age barriers to the competitive job market erected by societal conventions. To fell these barriers, Congress used the tried-and-true tool of a strong presumption against maximum age hiring categories. Against this presumption, however, the employer may vindicate his hiring policies by demonstrating that the contended discrimination is a bona fide occupational qualification that meets the Weeks-Diaz standard discussed in this opinion.
AFFIRMED.