Opinion for the court by Associate Judge KING.
Dissenting opinion by Associate Judge SCHWELB at p. 454.
KING, Associate Judge:
This appeal presents the question whether the setting of the timing of a traffic light by a municipal employee is a discretionary act which would make the municipality immune from liability for injuries proximately caused by the interval set. We. hold, under the circumstances presented here, that such an act is discretionary, thereby conferring immunity from liability on the District.
In this negligence action against the District for injuries resulting from an automobile striking a pedestrian in a crosswalk, the pedestrian, appellant Georges Aguehounde (“Aguehounde”), seeks reversal of the trial court’s judgment in favor of the District of Columbia (“District”) and reinstatement of the jury’s verdict in his favor. Aguehounde contends the trial court erred in granting the District’s motion for judgment as a matter of law on the ground that the District was immune from tort liability for the discretionary act of setting the timing of traffic light intervals. Aguehounde also maintains that the trial court erred in ruling that he was contributorily negligent as a matter of law for failing to look in the direction of the vehicle which struck him before he stepped into the crosswalk. For the reasons set forth below, we affirm the trial court’s grant of the District’s motion for judgment as a matter of law.
I.
At approximately 5:00 p.m. on April 28, 1990, Aguehounde was struck by a car driven by Erica Davis (“Davis”) after he stepped into the crosswalk on Fessenden Street as he walked north on the east side of Wisconsin Avenue (which runs in a north-northwest/south-southeast direction) in northwest Washington, D.C. The Davis vehicle, which was proceeding east on Fessenden St., (which runs in an east/west direction) struck Aguehounde in the crosswalk on the east side of Wisconsin Ave. Aguehounde testified that as he approached the Fessenden St./Wisconsin Ave. intersection, and when he was three or four steps from the southeast corner of that intersection, he looked to his left and did not see any vehicles approaching from the west on Fessenden Street. He further testified that when he reached the corner he saw that the light facing him was green and that cars, pointed in a westerly direction, were stopped to his right on Fessenden Street. He could not recall whether the “walk” or “don’t walk” sign was on. Aguehounde acknowledged that he did not stop at the corner before stepping into the crosswalk. He also stated that he did not remember looking to his left or seeing Davis’s car approach him from that direction as he stepped into the intersection.
Davis, the driver of the car which struck Aguehounde, testified that the light facing her turned from red to green when she was approximately one block from the Wisconsin Ave. intersection. She further testified that the traffic light was still green and she was traveling approximately 20 miles per hour as she proceeded through the intersection in her easterly course on Fessenden St. She first noticed Aguehounde standing on the curb as she passed through the intersection into the crosswalk. Aguehounde then took a big step out in front of her car while it was in the crosswalk. Davis testified that Ague-[445]*445hounde was looking away from her ear and she never saw him look in her direction. She applied her brakes, but nonetheless struck Aguehounde who landed 3-5 feet in front of her car.
As a result of the injuries sustained in the collision, Aguehounde and his wife brought this negligence action against the District,1 alleging that: the District failed to follow the proper engineering standards in setting the length of the “clearance interval” at the intersection; this failure caused the District to set a clearance interval of too short a duration to allow cars to clear the intersection and the crosswalks; and consequently, Davis’s car was still in the intersection when Aguehounde stepped into the crosswalk on a green light.2
A jury trial was conducted in May, 1993 before Judge Burgess. The District’s motion for judgment as a matter of law on the ground that it was immune from suit because the setting of the clearance interval was a discretionary act, was denied. The jury found that the District’s setting of the timing of the clearance interval was the proximate cause of Aguehounde’s paralyzing injuries, and awarded him $7,318,313.20 in damages, and his wife $602,913 on her claim for loss of consortium. In addition, the jury rejected the District’s contention that Aguehounde had been contributorily negligent.
Thereafter, the District moved for judgment as a matter of law on the grounds that the timing of the light is a discretionary function immune from tort liability, and that Aguehounde was contributorily negligent as a matter of law. In a fifty-five page Memorandum and Order, Judge Burgess granted the District’s motion, concluding that the District was immune from liability because the decision to set the length of the clearance interval involved policy considerations, and because the District had not adopted a specific directive taking away the traffic engineer’s discretion in establishing timing intervals. The trial court also ruled that Ague-hounde was contributory negligent as a matter of law in entering the intersection without taking reasonable steps to ensure his own safety.
On appeal, in No. 93-CV-1116, Aguehounde contends the trial court erred in granting the District’s Motion for judgment as a matter of law, alleging that: (1) setting the clearance interval is a ministerial act, devoid of policy considerations; (2) the District removed any discretion in the timing by adopting the national traffic formula; (3) the trial court’s factual finding on the immunity issue violated the Seventh Amendment;3 [446]*446and (4) Aguehounde was not contributorily negligent as a matter of law for entering the crosswalk.4
II.
Before turning to the merits, a brief discussion of practices relating to the timing of traffic lights will be helpful to understanding the issue presented. The clearance interval of a traffic light is the amount of time allocated to the yellow light which occurs between the green light for one street and the green light for the cross street at an intersection. At some intersections an “all-red” sequence is added, meaning there is a red light in each direction at the same time for the duration of that sequence, which is ordinarily no more than one or two seconds. The purpose of the clearance interval is: (1) to allow a driver sufficient time to either come to a complete stop before entering the intersection or to clear the intersection before the crossing light turns green; and (2) to keep opposing traffic and pedestrians from entering the intersection until the vehicles headed in the opposite direction have passed through the intersection.
It is undisputed that in 1985 the clearance interval at the Wisconsin Ave./Fessenden St. intersection for traffic coming from Davis’s direction was 4.5 seconds, meaning the light facing traffic on Fessenden St. would turn from green to yellow for 4.5 seconds before the light facing traffic on Wisconsin Ave. would turn to green. In October 1989, the interval was changed to 4.0 seconds.
The principal formula used by the District in setting the timing of traffic light intervals is contained in the chart “Required Yellow Interval In Seconds,” which provides:
Y = T + % V + (W + L)
A V
The factor “Y” is the clearance interval expressed in seconds. “T” represents the “perception reaction time,” which is the time it takes a driver to perceive the yellow light and react to it by moving his foot from the accelerator to the brake, and for braking to begin. The “accepted figure” for “T” is one second. The second part of the formula Qfi “V” divided by “A”) calculates the “accepted deceleration rate for a controlled stop” with “V” being the velocity of the vehicle, expressed in feet per second, and “A” representing the deceleration rate of a vehicle, expressed in feet per second per second. The speed limit at the intersection was 25 mph or 36.7 feet per second. Typical “A” values vary from 10, for a “fairly fast stop,” to 15 for a “more conservative, comfortable stop.” The third part of the formula computes the time it would take a car to clear the intersection. This is done by adding the width of the intersection (“W”) to the length of the car (“L”) and dividing by the velocity at which the car is traveling in feet per second (“V”). In general, 12 feet is used as the “L” for automobiles, and a larger number is used for longer vehicles such as trucks. In applying this formula, the greater the speed, the shorter the clearance interval and vice versa.
A six-second clearance interval5 involving four seconds of yellow and two of all-red, which Aguehounde contends should have been set for the intersection, would have allowed vehicles to clear the intersection and the crosswalks before the green light for opposing traffic and pedestrians would permit entry to the intersection, thus maximizing pedestrian safety. A four or 4.5 second clearance interval, however, might not necessarily permit all traffic to fully clear the intersection before pedestrians were permit[447]*447ted to enter the intersection and, consequently, traffic flow, but not pedestrian safety, would be maximized.
III.
A. Is Setting Traffic Light Intervals a Discretionary Function?
Under the common law, a municipality is immune from suit for decisions made pursuant to the exercise of discretion, but not for actions which are ministerial.6 McKethean v. WMATA 588 A.2d 708, 715 (D.C.1991); Elgin v. District of Columbia, 119 U.S.App. D.C. 116, 119, 337 F.2d 152, 155 (1964). Whether a function is discretionary or ministerial is a question going to the subject matter jurisdiction of the trial court. District of Columbia v. North Wash. Neighbors, Inc., supra note 3, 367 A.2d at 148 n. 7. It is a determination to be made by the trial judge, not the jury, and this court conducts a de novo review of the trial court’s determination of whether or not the action was discretionary. See e.g., id.; Daigle v. Shell Oil Co., 972 F.2d 1527, 1537, 1539 (10th Cir.1992). However, as is the case with any de novo review, “we must accept the trial court’s resolution of conflicting testimony and will not disturb the factual findings so long as they are supported by substantial evidence.” United States v. Alexander, 428 A.2d 42, 50 (D.C.1981) (reviewing trial court’s resolution of a motion to suppress tangible evidence).7 Further, when determining whether the act is discretionary, which determines whether the court has subject matter jurisdiction, the trial court is not confined to considering only that evidence which was also heard by the jury; instead, the court may consider all evidence coming to its attention bearing on that issue. See, e.g., Matthews v. Automated Bus. Sys. & Serv., 558 A.2d 1175, 1179-80 (D.C.1989) (“the court has broad discretion in determining how to proceed in finding such [jurisdictional] facts, including basing its decision on affidavits”).8
At the outset, the trial court must determine whether the act is a discretionary or ministerial function under the circumstances presented. McKethean, 588 A.2d at 715. While “[cjharaeterizing an act as discretionary or ministerial is not always an easy task,” id, discretionary acts are generally defined as those acts involving the formulation of policy while ministerial acts are defined as those relating to the execution of policy. See id.; Wade v. District of Columbia, 310 A.2d 857, 860 (D.C.1973) (en banc) (discretionary functions are those which, if liable in tort, would pose threats to the quality and efficiency of the government). Administrative decisions which require the government to balance competing considerations are considered discretionary acts. McKethean, 588 [448]*448A.2d at 715. By barring suit for such actions, Congress “prevent[s] judicial “second-guessing” of legislative and administrative decisions grounded in social, economic and political policy through the medium of an action in tort.” United States v. Varig Airlines, 467 U.S. 797, 814, 104 S.Ct. 2755, 2764, 81 L.Ed.2d 660 (1984).
To determine whether a given governmental action is discretionary or ministerial, we first determine whether it is the kind of action “that the discretionary function exception was designed to shield;” that is, whether the action involves “the permissible exercise of policy judgment.” Berkovitz v. United States, 486 U.S. 531, 536-7, 108 S.Ct. 1954, 1958-59, 100 L.Ed.2d 531 (1988) (citation omitted); McKethean, 588 A.2d at 715. If the answer to this first inquiry is yes, then the action is immune from suit, unless the government has adopted a “statute, regulation or policy [that] specifically prescribes a course of action for an employee to follow.” Berkovitz, 486 U.S. at 536, 108 S.Ct. at 1958; McKethean, 588 A.2d at 715. If such a specific directive exists which removes the otherwise unfettered discretion of the government employee, the action is ministerial, opening the government to suit if not performed correctly.
Aguehounde contends that the trial court erred in finding that the setting of the clearance intervals was a discretionary act because the decision did not involve policy considerations making it the type of action intended to be shielded by immunity. He further contends that, even if setting the timing was a discretionary act, the District is not immune because the District had adopted a specific formula which its employees were required to follow in setting traffic light intervals, thereby removing any element of choice the traffic engineers may have had.
It was clear to the trial court that setting traffic signal intervals “is the type of function that the discretionary function rule is designed to shield.” The trial court further observed that setting traffic signal intervals:
involves considerations of safety not only for pedestrians but for travellers, and it involves a balancing of safety needs against the need to assure adequate traffic flow, which itself involves considerations of safety as well as commerce and convenience. Balancing these factors also requires the ascertainment of facts, such as numbers of vehicles and pedestrians, and ways in which drivers and pedestrians behave in the aggregate, which are peculiarly subject to study and expertise. Subjecting the decisions of traffic engineers to litigation and to second-guessing by jurors would deter effective government.
We fully agree, and therefore hold, for the reasons relied upon by the trial court, that the timing of signal intervals involves balancing various economic, political and social considerations and is therefore a discretionary function.9 For example, the need to accommodate pedestrians may be paramount at some intersections, but of only secondary importance at other intersections. Thus, intersections with high pedestrian traffic might call for different considerations than those appropriate at intersections where there is little or no pedestrian traffic but the interest in expeditiously moving vehicular traffic predominates. These determinations are clearly judgment calls involving the application of policy considerations as well as a knowledge of the traffic flow needs of the area where the particular intersection is located. The timing interval is therefore one calling for an exercise of discretion.
Aguehounde maintains, nonetheless, that the timing of light intervals “cannot fairly be characterized as a [traffic] design decision,” and is thus not immune from suit under the [449]*449authorities cited above. Essentially, Ague-hounde contends that the design of the intersection had already been established through the decisions regarding the width of the streets, the permitted speed of the cars, and the installation of traffic lights. Consequently, he argues, the timing of the yellow light is merely an implementation of the pre-existing design and thus not discretionary. This argument misapprehends established law in this jurisdiction. For example, in McKethe-an we held that “the decision to relocate a bus stop ... is part of overall traffic and safety design,” McKethean, 588 A.2d at 715 (emphasis added), and in Urow we determined that “the placement of traffic control devices [is] one aspect of street design.” Pace, supra, 498 A.2d at 229 (citing Urow, supra note 9, 114 U.SApp.D.C. 350, 316 F.2d 351) (emphasis added).
Nor do we find any support for our dissenting colleague’s assertion that government conduct is insulated from liability only where the official “actually exercised discretion.” The dissent concedes that there is no dispositive District precedent supporting that view, and in fact, to the extent it has been addressed, the contrary is true. For example, in McKethean, supra, 588 A.2d at 715, we cited with approval the observation by the U.S. Supreme Court that “where there is room for policy judgment and decision, there is discretion.” Dalehite v. United States, 346 U.S. 15, 36, 73 S.Ct. 956, 968, 97 L.Ed. 1427 (1953). Consequently, an act is deemed discretionary where it is shown to involve policy judgment and decisions, but does not require proof that every myriad and intricate decision involved in the process required a specific balancing of policy decisions.
We conclude, for the reasons relied upon in McKethean and Urow, that the timing of the signal interval is simply one element of traffic design. To view the timing of a traffic light in a vacuum would ignore the effect that the timing of one interval has on the overall traffic pattern in the District. The setting of one light affects the timing and control of traffic on the entire street and its intersections, and can thus impact the city-wide traffic flow. The decision regarding the timing of the light is thus an integral part of overall traffic flow decisions and cannot be singled out.10 To subject one aspect of overall traffic design to judicial second-guessing would im-permissibly impact the District’s autonomy in regulating traffic flow. Therefore, because our authorities have squarely held that traffic design is a discretionary function and that the various aspects of such design are also discretionary, we hold that setting traffic intervals is likewise discretionary. See District of Columbia v. Pace, 498 A.2d at 229; McKethean, 588 A.2d at 715; North Wash. Neighbors, 367 A.2d at 148 n. 7.
Aguehounde also contends that the light interval decision “involved purely engineering calculations” requiring professional expertise, but not any policy considerations, thus making it a ministerial rather than a discretionary act. This claim is premised on the proposition that, to establish immunity, the government must produce evidence that “social, political or economic considerations entered into the timing of the clearance interval at Wisconsin and Fessenden.” Ague-hounde further argues that “both common sense and expert testimony ... dictate that once a decision is made to have a clearance interval ... the interval must be set long enough to allow traffic to clear the intersection.”
If we were to accept Aguehounde’s argument, the District would be required to justify the policy underlying each of the myriad decisions involved in traffic design. Our case law suggests, however, that the proper inquiry is not: what concerns were actually balanced in each individual act? Instead, we [450]*450should ascertain whether the type of function at question is grounded in policy analysis. See, e.g., United States v. Gaubert, 499 U.S. 316, 111 S.Ct. 1267, 113 L.Ed.2d 335 (1991). (“Discretionary conduct is not confined to the policy or planning level ... but on the nature of the actions taken and on whether they are susceptible to policy analysis”); Baum v. United States, 986 F.2d 716, 720-21 (4th Cir.1993) (court should examine whether “decision is one which we would expect inherently to be grounded in considerations of policy” and not conduct a “fact-based inquiry into the circumstances surrounding the government actor’s exercise of a particular discretionary function”). Appellant’s suggested approach of isolating each component of a decision is simply not required. See Smith v. Johns-Manville Corp., 796 F.2d 301, 308 (3d Cir.1986) (“The position that agency decisions can be broken down into component parts is fundamentally at odds with the Court’s teaching in Dalehite.”). Just as the length of yellow intervals is part of the overall traffic design, it is part of the overall policy of determining traffic flow in the District. See Daigle, supra, 972 F.2d at 1542 (“[a] decision that is a component of an overall policy decision protected by the discretionary function exception also is protected by this exception”).11
In support of his contention that the government must establish that setting traffic intervals involved policy considerations, appellant relies on WMATA v. O’Neill, 633 A.2d 834 (D.C.1993). That reliance is misplaced. In O’Neill, the court held that a bus driver’s inaction in dealing with disruptive passengers was not discretionary because there was an established policy in place indicating the safety rules the driver was to follow in such situations. O’Neill, 633 A.2d at 839. We held that WMATA was not immune from suit grounded in the alleged negligence of the driver in carrying out specific directives. Id. Aguehounde correctly reasons that this holding means that the District would not be shielded from liability for the negligence of an employee who fails to follow a set policy.
Appellant misreads O’Neill, however, when he contends that it stands for the proposition that there must be evidence of policy considerations in the employee’s decision-making before immunity attaches. O’Neill does no more than hold that the District would be liable where an employee fails to follow an established policy, because the existence of a set policy means all discretion has been removed from the employee, and the employee’s actions would thus be ministerial. O’Neill does not hold, nor suggest, that the government must prove the specific policy considerations behind each aspect of a given action. Consequently, O’Neill does not advance Aguehounde’s contention that, because setting traffic intervals may involve engineering calculations requiring professional expertise, the District is required to prove the policy considerations behind the timing decisions to establish that the action is discretionary.12
[451]*451
B. Did the District Adopt a Specific Directive Mandating the Setting of Traffic Intervals'.?
Finding that the setting of yellow intervals is a discretionary function does not end our inquiry. We now must determine whether there were any specific directives withdrawing from employees the option of exercising choice, which would transform the setting of the interval timing from a discretionary to a ministerial task. McKethean, 588 A.2d at 715; Berkovitz, supra, 486 U.S. at 586, 108 S.Ct. at 1958 (“the discretionary function exception will not apply when a federal statute, regulation, or policy specifically prescribes a course of action for an employee to follow”). In the instant case no statute or regulation exists prescribing what clearance interval the Signal Design Branch of the District of Columbia Department of Transportation must set for the intersection. The only possible “directive” is the chart labelled “Required Yellow Interval in Seconds” (“chart”), and we must therefore determine whether that chart constitutes a “specific mandatory directive” for employees to follow in setting the interval timing which would render implementation by employees ministerial.
Aguehounde contends that the chart removed all room for the exercise of a policy judgment by mandating a particular formula that the engineers must use to set clearance intervals at intersections. In support of this argument, Aguehounde relies on the District’s answer to Interrogatory No. 113 and the word “required” in the title of the chart as evidence that “the District had adopted a policy of following the national formula.” Alternatively, appellant argues that even if the evidence did not prove the District had adopted the national timing formula, the interrogatory answers proved that it did so at the intersection in question and mis-mea-sured the width, which was “an indisputably ministerial act.” We conclude that the evidence considered as a whole did not establish the existence of a mandatory policy for setting traffic intervals which would transform the function into a ministerial one. We also conclude that any mis-measurement, if there was one, was therefore irrelevant in this analysis.
[452]*452Our review of the evidence confirms the trial judge’s findings that there was no policy or specific directive mandating that District engineers follow the formula contained in the chart. On this point the trial judge found there was no evidence “that the chart was issued as part of any memorandum or other writing directing that traffic engineers use it.... Nor is there evidence of any oral communication requiring its use.” Indeed, the testimony of the chief engineer established that there was no written policy requiring use of the chart and that an engineer uses that formula “because he feels that it is the chart he should use.” The trial judge credited this assertion of the chief engineer and placed great reliance upon it, noting that the fact that an engineer uses the chart not out of obligation, but because he “feels he should,” is not sufficient, without more, to establish that the District mandated its use. Because we conclude that the trial judge was not clearly erroneous in crediting the engineer’s testimony, we defer to his findings of fact. See, e.g., Alexander, supra, 428 A.2d at 50; In re Baby Boy C, supra, 630 A.2d at 683.
Moreover, the testimony at trial established that there were several different charts which the engineers used to determine the length of signal intervals.14 The trial judge’s findings of fact, which have ample support in the record, confirms that several formulas were available for the District engineers to choose from in setting light intervals, and that there was no directive of any kind requiring an engineer to use the chart. In sum, the evidence supports the trial court’s finding that there had been no showing that the District had adopted the formula in the chart as a mandatory directive.
Nonetheless, Aguehounde argues that “the very [use of the] “required” language in the chart’s title amply proved th[e] point” that the chart was a required directive. The evidence is to the contrary, and as the trial court found, without more, “the word required ... does not itself establish a specific directive that the Signal Design Branch change the interval at Fessenden and Wisconsin .... ”
Appellant’s alternative argument that Interrogatory No. 1, see supra note 13, establishes a mandatory policy for setting the signal intervals must also be rejected. The interrogatory answer does not establish that the District had adopted a mandatory directive to use the formula. At most, the response shows that an engineer used a specific formula in setting the interval at the intersection in question. It does not establish that the District required him to do so.
Finally, Aguehounde contends that the engineer’s mis-measurement of the width of the intersection is a ministerial act for which the District can be liable if the asserted mis-measurement can be linked to the injury. Aguehounde claims the District failed to include both crosswalks in its measurement which resulted in a calculation of 65 feet for the width of the intersection. At trial, Ague-hounde’s traffic engineering expert, Sheldon Pivnik, testified that the width of the intersection, “W”, in the clearance interval formula used by the District (see discussion, supra at 446-447), must be determined by measuring from the stop line beyond the crosswalk at one side of the intersection to the corresponding stop line at the other side of the intersection. Using that definition of “W”, the width of Wisconsin avenue is 106 feet. Thus argues Aguehounde, because the clearance interval was originally set at 4.5 seconds at the Fessenden/Wiseonsin intersection, the District must have mis-measured the width since a “W” value of 65 feet, rather than the [453]*453correct 106 feet, would result in an interval of 4.5 seconds.
The government maintains, however, that using 65 feet as the “W” in the formula results in an interval of 4.9 seconds rather than 4.5 seconds.15 Therefore, it contends the 4.5 second interval could not have resulted from the claimed faulty mis-measurement. Furthermore, the District could have, in the exercise of its discretion, in order to minimize traffic delays, set the interval to accommodate motorists rather than pedestrians. That would be the result with a 4.0 or even a 4.5 second clearance interval.
Finally, with respect to the so-called mis-measurement, our dissenting colleague reads far more into the testimony of Saraj Gyani, a District traffic engineer, than the trial judge did or we do. Judge Schwelb asserts that the witness “effectively admitted ... that ... it was a faulty measurement of the intersection, rather than a high-level choice between policies, that led to ... an erroneous ... clearance interval_” That assessment significantly overstates what the witness said. Gyani was presented with an exhibit16 that reflected the results of the calculation of the interval timing, apparently pursuant to the formula found in the “Required Yellow Interval in Seconds” chart discussed supra at 446; 451, for various clearance distances expressed in feet.
We think Gyani’s testimony, given in response to a series of leading questions, can fairly be summarized as follows:
If that formula is followed, and if it is intended to set the interval to ensure that the pedestrian crosswalks are cleared, the width used should be as measured from the outer limits of the crosswalk on one side of the street to the corresponding part of the crosswalk on the other side of the street which, in this case, is just over 105 feet, then the clearance interval, according to the exhibit would be 5.7 seconds.
The trial judge, who heard the witness’s responses to counsel’s questions, concluded, as we do, that the testimony was “intended to mean, if the chart were used, the calculations show that the interval should be six seconds.” In short, although the numbers do not precisely jibe, that testimony does no more than confirm, the calculation by Aguehounde’s expert Clyde Richard as set forth in note 5, supra. Gyani’s testimony on this point does not advance Aguehounde’s cause, however, because it in no way contradicts the testimony, credited by the trial court, that the formula was not mandated by the District or even used at that intersection. As we have said, because use of the formula was not mandated by the District, any error in applying it, if there was one, does not convert a discretionary act into a ministerial one.
The question of negligence has no relevance until it is established that an act was ministerial. See McKethean, supra, 588 A.2d at 715; O’Neill, supra, 633 A.2d at 837-38; Johnson v. U.S. Dept. of Interior, 949 F.2d 332, 340 (10th Cir.1991) (“[Fjactual issues [454]*454concerning negligence are irrelevant to the threshold issue whether the officials’ actions are shielded from liability by the discretionary function exception”) (citation omitted). Consequently, Aguehounde’s argument that the District mis-measured the width of the intersection does not assist us with the determination of whether the clearance interval was a discretionary or ministerial function. If we had first found that setting intervals was ministerial, then consideration of the alleged mis-measurement might be probative of the negligence of the District’s employees in performing this ministerial act. See Cope v. Scott, 310 U.S.App.D.C. at 147, 45 F.3d at 448. (“If a specific directive exists ... [t]he only issue is whether the employee followed the directive”) (emphasis added). Because we have determined that setting the intervals is a discretionary act, however, and because the District did not mandate the use of the formula in question, any alleged mis-mea-surement of the intersection is immaterial, because the District is not liable for negligently performed discretionary acts.
In sum, because the record supports the finding that the District had no mandatory policy in place for determining signal intervals, the trial court properly could find, as it did, that the District used the formula in the chart as a guide, therefore making its use a discretionary function.
No. 93-CV-1116 Affirmed.
No. 93-CV-1213 Dismissed as Moot.