For more than 30 years, law enforcement has relied on the Precision Immobilization Technique (PIT) to disable fleeing vehicles and to remove aggressor vehicles from motorcades or convoys. When conducting a PIT maneuver, an officer applies lateral pressure to the rear quarter panel of the target vehicle so it spins out and stops. The PIT maneuver requires precision to produce this outcome, and, traditionally, the PIT maneuver has been effective because vehicle mechanics and physics make the outcome predictable. However, advances in automotive technology—particularly the introduction of Electronic Stability Control (ESC) systems—bring into question the reliability and effectiveness of the PIT maneuver for modern vehicles.
The National Highway Traffic Safety Administration began mandating ESC systems in new cars in 2012; however, ESC has been available as an optional feature since 1988.1 ESC is an onboard safety technology designed to proactively help drivers maintain control of their vehicles in situations where the vehicle is beginning to lose directional stability. Typically, an ESC system intervenes by utilizing computers to control individual wheel brakes, thereby helping to keep the vehicle headed in the direction intended by the driver.2 ESC systems vary by manufacturer and are marketed under different names such as Stabilitrak, Electronic Stability Program, and Dynamic Stability Program.
Previous research on the PIT revealed potential differences in vehicle response by vehicles equipped with ESC, leading researchers to express concern that ESC could make the PIT less effective.3 However, only one known study, conducted by the Portland (Oregon) Police Bureau directly assessed the effect of ESC on the PIT.4 Key findings included the following:
• When conducting a PIT maneuver, secondary impacts were more common in target vehicles with ESC than in vehicles without ESC. Secondary impact occurs when the force of the initial impact causes the target vehicle to spin around and collide with the PIT vehicle.
• The severity of secondary impacts increased at higher speeds (45+ mph or 72.4+ kph).
• Inconsistencies in performance existed between different vehicle makes (e.g., Ford, Chevrolet, Dodge).
• Researchers found no evidence that an ESC-equipped target vehicle can rotate 360 degrees and continue traveling in the same direction.
These findings provided initial evidence that the application of PIT might need to change to account for ESC, which would require a change in training. To meet the need to identify an effective PIT for ESC-equipped vehicles and then to adjust training based on these findings, the Federal Law Enforcement Training Centers (FLETC) Driver and Marine Division conducted research to determine how vehicles equipped with ESC react to the PIT as compared to vehicles not equipped with ESC. Specifically, the goals of the research were to determine if law enforcement could safely and effectively perform PIT maneuvers on ESC vehicles and to identify how ESC vehicles respond to PIT. Based upon the research findings, the goal was to update training curriculum so that students would factor in ESC when performing a PIT and adjust their technique to achieve an optimal outcome.
The PIT Study
FLETC researchers collected data using an ESC-equipped vehicle (PIT vehicle) to conduct the PIT maneuver on ESC-equipped vehicles (ESC target vehicle) and non-ESC vehicles (non-ESC target vehicle). Each target vehicle completed 10 repetitions on a 1.5-mile asphalt road course at 25 mph (40.2 kph), 35 mph (56.3 kph), and 55 mph (88.5 kph).
To collect vehicle performance data, researchers used Video Vboxes and DriftBoxes produced by Racelogic, Ltd. The Video Xbox captured visual data from the perspective of the PIT and target vehicles. Cameras were located on the front windshield, rear window, driver’s side window, and ESC warning light area. The DriftBox recorded vehicle performance, such as drift angle, g-force, speed, distance, and acceleration. Figure 1 shows the configuration of data collection devices inside one of the test vehicles.
Data analysis consisted of comparisons between an ESC target vehicle and non-ESC target vehicle performance on (1) PIT vehicle: steering wheel angle, PIT accelerator, and brake pedal pressure and (2) target vehicle: centerline deviation, lateral acceleration, and yaw rate (degrees per second that a vehicle rotates on its vertical axis).
Vehicle Performance Factors
Steering Wheel Angle: Steering wheel position angle and degree of turn.
Accelerator Pedal: Amount of acceleration input given to the pedal in percentages.
Brake Pedal: Amount of brake input given to the pedal in percentages.
Centerline Deviation (in feet): Ability to track in a straight line under the influence of external conditions such as side wind susceptibility, road camber, suspension geometry errors. By knowing the heading of “straight ahead,” the software can determine any deviation from the reference line.
Lateral Acceleration: lateral force acting on a vehicle sideways to the direction of travel. For example, it is noticeable as centrifugal force moves a vehicle to the outside of a curve when cornering.
Yaw: degrees per second that a vehicle is rotating about its vertical axis.
Positive PIT: target vehicle is outside of the PIT vehicle in a turn.
Negative PIT: target vehicle is inside of the PIT vehicle in a turn.
ESC-Equipped Vehicle Performance Differences
Results of the FLETC study indicate differences in PIT vehicle and target vehicle performance and PIT outcomes based on the presence of ESC.
PIT Vehicle Data
The PIT vehicle required greater steering input with ESC vehicles, as measured by steering wheel angle and degree of turn. This difference was greater at slower speeds (see Figure 2). In addition, PIT vehicles needed greater acceleration to accurately deploy the technique on ESC target vehicles. These differences occurred across all vehicle speeds and vehicle components of accelerator pedal and brake pedal (see Figures 3 and 4). For the ESC target vehicle, PIT drivers needed to brake harder during the PIT follow-through to avoid secondary impact, in contrast to a non-ESC target vehicle.
During data collection, researchers noted that the ESC target vehicle’s brake lights occasionally came on when the driver was not pressing the brakes, which caused the PIT drivers to back off the PIT maneuver. The researchers discovered that, in some vehicles, the brake lights can come on during ESC activation without brake pedal application. They also found that the internal ESC notification light would occasionally come on in the PIT vehicle, indicating ESC activation, when making contact with the target vehicle.
Target Vehicle Data
The study revealed a difference in the centerline deviation between the ESC target vehicle and non-ESC target vehicle (see Figure 5). The non-ESC target vehicle travelled farther from the centerline than did the ESC target vehicle. This finding indicated that when officers use PIT, they should be prepared for ESC-target vehicles to come to rest closer to the PIT vehicle, thus reducing officers’ reactionary gap. Generally, when non-ESC target vehicles spin out, their engines stall out. During this study, the ESC target vehicle engine stalled less often than did the non-ESC target vehicle engine.
Researchers also found that the non-ESC target vehicle experienced greater lateral acceleration compared to the ESC target vehicle (see Figure 6).
Data showed that the ESC target vehicle had an increased yaw rate compared to the non-ESC target vehicle. This finding seemed inconsistent with observations that the ESC target vehicle appeared to be rotating slower. When considering the yaw rate in conjunction with the centerline deviation data, the researchers realized the target vehicle rotated in a tighter circle, which explains why an officer may perceive a slower than actual rotation of an ESC target vehicle.
In a follow-up study, FLETC gathered data to test the effects of the PIT on ESC–equipped SUVs to explore potential differences in performance when vehicles have a high center of gravity (HCGV). Given the practical importance of this data, this report includes preliminary findings.
The data gathered for target HCGVs under the same research conditions revealed similar outcomes as shown in the initial study of sedan target vehicles. ESC target HCGVs and non-ESC target HCGVs showed the same pattern of performance differences as the sedans.
Positive and Negative PIT (Performing a PIT in a Turn)
Positive PIT: Researchers found that although there was a high PIT success rate when the ESC target vehicle was outside of the PIT vehicle in a turn, the PIT vehicle frequently left the roadway. This was a likely result of the additional steering input needed to PIT an ESC target vehicle, combined with the vehicle’s position in the turn. Officers should be discouraged from performing a positive PIT on ESC target vehicles and must be aware that if they do so, they must be prepared to perform an off-road recovery.
Negative PIT: This research showed a low success rate when the ESC target vehicle was inside of the PIT vehicle in a turn. In general, the specific year, make, model, and options of the target vehicle can influence the success rate of the PIT, and it is not reasonable for officers to assess all relevant variables in a pursuit or PIT scenario. Therefore, we do not recommend that officers attempt a negative PIT. If officers attempt a negative PIT of an ESC target vehicle, there is a high likelihood of the attempt failing.
These findings demonstrate that, when conducting a PIT maneuver, officers should increase steering input, acceleration, and braking to successfully use PIT on an ESC-enabled target vehicle. Officers also need to be aware that the ESC on PIT vehicles may activate during PIT attempts, which could adversely affect outcomes. Further, a secondary impact is more likely to occur when using the PIT on an ESC target vehicle because the vehicle will not rotate as far as a non-ESC target vehicle. Additionally, ESC target vehicles might come to rest closer to the PIT vehicle than non-ESC target vehicles, thus reducing an officer’s reactionary gap. During the study, researchers found a lower frequency of stalling out by the ESC target vehicle following PIT. This is likely because ESC and Powertrain Control Modules (PCM) tend to cut power without stalling the vehicle. When conducting the PIT on HCGVs, officers should expect similar outcomes when for ESC targets. However, officers should be aware that the steering response in the HCGV PIT vehicle may be slower and the visual targeting may be more difficult.
Data from this study indicated that changes to PIT training curriculum are necessary. FLETC recommends the following additions to instruction to account for ESC vehicles:
• Stress the reduced “sweet spot” in both ESC target and PIT vehicles.
• Increase emphasis on sound four-corner awareness in the PIT vehicle.
• Highlight awareness of brake light activation in the ESC target vehicle.
• Recognize ESC light activation indicating ESC activation in the PIT vehicle.
• Emphasize the PIT vehicle may perform differently when ESC activates, which may increase driver anxiety or errors.
• Ensure matching PIT bumper heights during training with HCGVs. During a training exercise, a fabricated PIT bumper of a sedan slid under the PIT bumper of the target HCGV. This caused the target HCGV to lift its inside rear tire off the ground, which made the target HCGV become unstable. As a result, the FLETC modified the fabricated HCGV PIT bumper with an additional bracket. This modification resolved the issue for the training vehicles.
Modern-day motor vehicle technologies become more sophisticated each year. As a result, some of these safety features may impact the way the PIT maneuver is performed and taught. As these technologies evolve, future studies will need to continue investigating their effect on the PIT maneuver and other law enforcement driving tactics and techniques.
1 Anders Lie et al., “The Effectiveness of Electronic Stability Control (ESC) in Reducing Real Life Crashes and Injuries,” Traffic Injury Prevention 7, no 1 (2006): 38–43.
2 U.S. Department of Transportation, National Highway Traffic Safety Administration, FMVSS No. 16: Electronical Stability Control Systems, Final Regulatory Impact Analysis (Washington, DC: Officer of Regulatory Analysis and Evaluation, 2007).
3 Jing Zhou, Jianbo Lu, and Huei Peng, “Vehicle Dynamics in Response to the Maneuver of Precision Immobilization Technique,” Proceedings of 2008 ASME Dynamic Systems and Control Conference (2008), 577–584.
4 Tracy Burleson et al., Effects of Electronic Stability Control on the Pursuit Intervention Technique (Portland, OR: Portland Police Bureau, 2015).