dissenting on rehearing en banc.
The majority opinion concludes that the University failed to sustain its burden to prove the affirmative defense of immunity because the appellees’ affidavits do not adequately address the need and risk aspects of good faith. I disagree with this conclusion in three respects.
Need Aspect
A police officer acts in good faith in a pursuit case if a reasonably prudent officer, under the same or similar circumstances, could have believed that the need to immediately apprehend the suspect outweighed a clear risk of harm to the public in continuing the pursuit. See City of Lancaster v. Chambers, 883 S.W.2d 650, 656 (Tex.1994). The “could have believed” aspect of this good faith test means that, in order to be entitled to summary judgment, an officer must prove that a reasonably prudent officer might have believed the pursuit should have been continued. See id. at 656-57. It does not mean that an officer has to prove that it would have been unreasonable to stop the pursuit; nor must the officer prove that all reasonably *716prudent officers would have continued the pursuit. See id. at 657.
Good faith depends on how a reasonably prudent officer could have assessed both the need to which an officer responds and the risks of the officer’s course of action, based on the officer’s perception of the facts at the time of the event. See Wadewitz v. Montgomery, 951 S.W.2d 464, 467 (Tex.1997). The “need” aspect of the test refers to the urgency of the circumstances requiring police intervention. See id. In the context of an emergency response, need is determined by such factors as the seriousness of the crime or accident to which the officer responds, whether the officer’s immediate presence is necessary to prevent injury or loss of life or to apprehend a suspect, and what alternative courses of action, if any, are available to achieve a comparable result. See id.
The “risk” aspect of good faith refers to the countervailing public safety concerns: the nature and severity of harm that the officer’s actions could cause (including injuries to bystanders as well as the possibility that an accident would prevent the officer from reaching the scene of the emergency), the likelihood that any harm would occur, and whether any risk of harm would be clear to a reasonably prudent officer. See id. Without taking both sides of the Chambers balancing test into account, an affiant cannot have a suitable basis for concluding that a reasonable officer could or could not have believed that the officer’s actions were justified. See id.
Conclusory statements by an expert witness are insufficient to support or defeat summary judgment. See id. at 466. Thus, an expert witness’ conclusory statement that a reasonable officer could or could not have taken some action will neither establish good faith at the summary judgment stage nor raise a fact issue to defeat summary judgment. See id. Instead, expert testimony on good faith must address what a reasonable officer could have believed under the circumstances and must be substantiated with reference to each aspect of the Chambers balancing test. See id. at 466-67.
In this case, Clark’s brief adequately challenges the sufficiency of appellees’ summary judgment proof as to the “need” aspect of the Chambers test:
Here there was no immediate need to continue pursuit of Co-Defendant Kevin Thomas. There was no substantial evidence to believe Kevin Thomas was actually involved in a fight or needed to be detained. In addition, it was not immediately necessary to apprehend Kevin Thomas, and any benefit it would have been to apprehend him immediately was greatly outweighed by the risk of harm posed to the public, the police officers involved, and Kevin Thomas himself in attempting to stop him.
With regard to need, Williams’ affidavit states:
I was dispatched to a fight in progress at the University Center Circle at approximately 2:00 A.M. Prior to my arrival at the scene, uniformed foot patrol officers Adams and Marquez attempted to detain the driver of a green Suzuki jeep who was suspected to have been involved in the fight. Officer Adams noticed the driver had a large bump on his head. He had his hands in the vehicle through the open window to check the condition of the driver. The driver (later determined to be Kevin Wayne Thomas) started his vehicle. Both officers repeatedly ordered him to stay at the scene. Officer Marquez also had his hands inside the vehicle trying to stop the driver from leaving the scene. The driver suddenly drove off. Officer Adams was able to extract his arms from the vehicle, but the right forearm of Officer Marquez was struck by the window frame. Officer Adams broadcast that the vehicle needed to be stopped.
[[Image here]]
When I made the decision to stop the green Suzuki jeep, I knew that the driver was a suspect in assaultive conduct and that he fled police officers who were attempting to detain him and had committed the criminal offense of evading detention which is a class B misdemeanor. Furthermore, the suspect had not been identified before he fled the foot patrol officers....
*717The manner in which the suspect operated his vehicle and the high rate of speed at which he traveled caused me to believe that the suspect posed a danger to the public.
Stewart’s affidavit similarly states:
While I was ordered by my supervising sergeant, [Williams], to stop the suspect vehicle, I was also aware that the driver of the speeding vehicle was believed to have engaged in a fight on the University campus and evaded detention by University police officers — such offense was, at least, a class B misdemeanor and pursuit was authorized by University of Houston policy. In addition, the speed at which the suspect passed my patrol unit was a clear indication that the driver of the suspect vehicle had no regard for the safety of the public and needed to be stopped.
The affidavits thus addressed the seriousness of the crime to which the officers responded and the officers’ perception of the urgency of the circumstances requiring police intervention. Although the affidavits do not necessarily establish that all reasonably prudent officers would have concluded that Thomas needed to be pursued, they set forth facts and reasons sufficient to show that a reasonably prudent officer might have believed that Thomas needed to be pursued. See Chambers, 883 S.W.2d at 656-57.
The majority opinion concludes that the affidavits do not sufficiently address the need aspect because neither “discusses the seriousness of the assault Thomas allegedly committed or what alternative courses of action, if any, were available to apprehend him.” Whether or not the affidavits address the seriousness of the original assault of which Thomas was suspected, they clearly address the seriousness of his actions in evading arrest and his assaultive conduct toward the officer whose arms were inside Thomas’ vehicle when he drove away.
Moreover, in stating that the need aspect is determined by “such factors as ” those found lacking by the majority, Wadewitz does not require affidavits to discuss every facet of need or risk, but only to address both need and risk and to do so “based on the officer’s perception of the facts at the time of the event.” See Wadewitz, 951 S.W.2d at 467 (emphasis added). Thus, if an officer’s perception of the facts at the time of the event happens to focus on considerations other than or fewer than those recited in Wadew-itz, a showing of good faith should not thereby be arbitrarily precluded if the urgency of the circumstances requiring police intervention is otherwise shown. Therefore, contrary to the majority opinion, I believe Williams’ and Stewart’s affidavits adequately addressed the need aspect of the Chambers balancing test.
Risk Aspect
Unassigned Error
A court of appeals may not reverse a trial court’s judgment in the absence of properly assigned error. See, e.g., Pat Baker Co., Inc. v. Wilson, 971 S.W.2d 447, 450 (Tex.1998); Vawter v. Garvey, 786 S.W.2d 263, 264 (Tex. 1990). A point of error is sufficient if it directs the attention of the appellate court to the error about which the complaint is made. See Tex.R.App. P. 74(d).1 In civil cases, complaints that the evidence is legally insufficient to support a particular issue or finding may be combined under a single point of error if the argument under the point sufficiently directs the court’s attention to the nature of the complaint made regarding each such issue. See id. Thus, for example, a point of error asserting broadly that the trial court erred by granting the summary judgment will allow an appellant to argue all reasons that the trial court erred in granting the summary judgment. See Star-Telegram, Inc. v. Doe, 915 S.W.2d 471, 473 (Tex.1995). Importantly, however, the argument in the brief must still specifically attack each particular ground and present such a discussion of the relevant facts and authorities as are necessary to sustain each challenge. See Tex. R.App. P. 74(f).
These principles mean that an appellate court is not free to reverse a judgment on *718any valid ground it may be able to uncover, but only on the grounds that have actually been presented by the appellant. To do otherwise would defeat the impartiality of the court by making it an advocate for the appellant.
In this case, Clark’s sole point of error states “The trial court erred in granting Defendant’s motion for summary judgment because there are genuine issues of material fact and Defendant is not entitled to judgment as a matter of law.” Among other things, the brief states that the defendants failed to prove the affirmative defense of immunity and that the defendants were not acting in good faith in deciding to engage in the pursuit. In addition, as indicated in the passage quoted above, the brief fairly challenges the sufficiency of the evidence of the need aspect.
However, the closest the brief comes to challenging the sufficiency of the evidence of the risk aspect are the statements: (1) “it was not immediately necessary to apprehend Kevin Thomas, and any benefit it would have been to apprehend him immediately was greatly outweighed by the risk of harm posed to the public, the police officers involved, and Kevin Thomas himself in attempting to stop him”; and (2) “In addition, it [the affidavit of Clark’s expert witness] proclaims that any reasonably prudent law enforcement officer, under the same or similar circumstances, could not have believed the need to immediately apprehend Mr. Thomas outweighed a clear risk of harm to other members of the public using the highway.” These statements clearly argue that the need was outweighed by the risk, but do not challenge the sufficiency of the evidence setting forth the risk.2 The majority opinion nevertheless concludes:
[T]he officers’ affidavits are not substantiated with sufficient reference to the risk aspect of the Chambers balancing test. Although the affidavits state the risk of pursuing Thomas was “diminished,” neither affidavit details the nature and severity of harm the pursuit may have caused or whether a reasonably prudent officer would detect any risk of harm.
I do not believe that any statements in Clark’s brief can fairly be read to challenge the sufficiency of the evidence supporting the risk aspect of good faith and thereby support this conclusion. Nor do' I believe that we are authorized to address it based only on a global challenge to the sufficiency of the evidence generally, as contrasted from a challenge to the sufficiency of the evidence of the risk aspect specifically. Therefore, I believe the majority’s conclusion reverses the summary judgment on a ground not properly before us. See Central Educ. Agency v. Burke, 711 S.W.2d 7, 9 (Tex.1986).
Evidence of Risk Aspect
Even if a proper challenge had been made with regard to the risk aspect, Williams’ affidavit states:
The weather was clear and the pavement was dry and the traffic was light due to the time of night. I was familiar with the area and streets upon which I was traveling. The area of Calhoun Street was basically commercial and I observed no cars other than the suspect vehicle and officer Stewart’s vehicle on the roadway. When the suspect vehicle proceeded onto M.L.K. Blvd., I knew that the street was divided by a median and that to the left was a green belt and to the right was a park; therefore, no traffic would be intersecting M.L.K. Blvd. There was no traffic from the point of entrance one (1) to M.L.K. Blvd. and no traffic from M.L.K. to Old Spanish Trail. From my professional experience, I was aware that suspect vehicles will often appear to flee but stop after having time to observe emergency lights and siren behind them. When I decided to pursue the suspect vehicle, I believed that at that time of night with the minimal traffic on the street and the nature of the area in which I traveled, diminished [sic] the risk from the pursuit itself, while the suspect and the *719manner in which he operated his vehicle posed a significant risk to the public.[3]
Because this portion of the affidavit addresses the prevailing road, traffic, and weather conditions and thus the likelihood harm would occur from the officer’s actions in this case, I believe it presents sufficient evidence of the risk aspect to support a conclusion that a reasonable officer might have believed Williams’ actions were justified based on the officer’s perception of the facts at the time of the event.
The majority opinion concludes that the affidavits did not establish the risk aspect because neither detailed “the nature and severity of harm the pursuit may have caused (including potential injuries to bystanders as well as the possibility that an accident would prevent the suspect from being apprehended) or whether a reasonably prudent police officer would detect any clear risk of harm.” Obviously, even under the most ideal circumstances, every situation in which vehicles are driven on roads presents some possibility that a motorist will fail to exercise ordinary care and cause damage or injury. This likelihood increases in pursuit situations due to higher speeds and the willingness of a suspect to take greater risks in order to escape. There is also always a possibility that another vehicle or pedestrian will appear during a chase unexpectedly and without adequate time to enable a chase vehicle to avoid injury or damage. Such abstract risks are inherent in any pursuit situation, and, if they alone outweighed the need to apprehend suspects, good faith could never be shown. Thus, no useful purpose is served by requiring police officers to reiterate such inherent risks in their affidavits just for the sake of doing so.
Instead, what should influence an officer’s decision in a particular case and what therefore should be described in his affidavit are the specific circumstances present that could increase the chance of damage or injury from a pursuit above what is inherent in any pursuit. In a given case, these might be, in type and number, comparable to or different from those outlined in Wadetvitz. Where specific risk factors are present in a case, whether of the type and number identified in Wadeuritz or otherwise, and the officers fail to perceive them or address them in their affidavits, the risk element of the good faith test is not met. On the other hand, where those risk factors are perceived by the officers and addressed in their affidavits, the risk element of the good faith test should be satisfied. If, for example, road, traffic, and weather conditions happen to be the only such factors which are pertinent in a particular case, the officers’ affidavits should not be held insufficient for failing to discuss others that did not exist.
In this case, the majority holds that evidence of road, traffic, and weather conditions is not sufficient to satisfy Wadewitz even though the record contains no allegation or evidence that any other risk factor was present and could have been perceived by the officers. An officer reading the majority opinion will thus be left to wonder whether, in addition to describing the risk factors that were actually presented, his affidavit must also: (a) list risk factors which are inherent in every pursuit, and (b) affirmatively negate the existence of all other risk factors that were not present. If so, a reason can always be found by a court to reject an affidavit as insufficient and deny summary judgment. Of greater concern, however, is whether such uncertainty about establishing good faith will interfere with the spontaneous and instinctive manner in which pursuit decisions must realistically be made.
Because I therefore believe that neither Clark’s challenge to the legal sufficiency of the evidence nor her remaining arguments can properly be sustained, I would affirm the judgment of the trial court.
ANDERSON and HUDSON, JJ., join this dissenting opinion.. Because this appeal was perfected before September 1, 1997, it is generally governed by the Texas Rules of Appellate Procedure in effect before that date.
. Nor is il apparent how the risk could be great enough to outweigh the need if it is not shown by sufficient evidence in the first place.
. Similar testimony is found in Stewart's affidavit.