DUI and Vehicular Homicide Reconstruction: Impairment, Causation, and Criminal Case Evidence

Felony DUI and vehicular homicide cases — in South Carolina, felony DUI (S.C. Code § 56-5-2945) and reckless vehicular homicide (§ 56-5-2910) — sit at the intersection of traffic collision reconstruction and criminal law. In a criminal case, the physical evidence is tested against the highest burden of proof. The analysis must address causation with precision, because the central question is often whether the driver’s conduct — not just impairment, but the specific operational choices the driver made — caused the death or serious injury at issue. And the reconstruction will face rigorous cross-examination, because the liberty of the accused and the justice owed to the victim depend on it.

Gerald McDevitt has worked DUI and vehicular homicide cases for both prosecution and defense across state and federal courts. The methodology is the same regardless of the retaining party: rigorous scientific analysis of the physical evidence, careful distinction between what the evidence establishes and what it does not, and a methodology developed to meet Daubert and Frye admissibility standards, with findings that can be examined under oath.

The Distinct Demands of DUI and Vehicular Homicide Cases

The reconstruction of an impaired-driver collision differs from a routine traffic reconstruction in several important respects. The burden of proof in a criminal case is substantially higher than in civil litigation. Because the prosecution must prove causation beyond a reasonable doubt, the reconstruction’s treatment of causation is examined closely. The interaction between impairment and driver conduct must be addressed carefully: the expert must avoid conflating the fact of impairment with the question of causal contribution. And the record must be built with the expectation that every assumption, every measurement, and every calculation will be challenged.

Prosecutors need to know whether the physical evidence supports their causation theory. Defense counsel need reconstructions that test the prosecution’s theories against the actual physical evidence, identify alternative explanations the scene evidence may support, and provide an independent test of the state’s reconstruction against the physical evidence. Both sides benefit from an expert whose findings are scientifically grounded rather than adversarial in posture.

Separating Impairment from Causation

One of the most important analytical distinctions in a DUI or vehicular homicide case is the difference between impairment and causation. The fact that a driver was impaired does not, by itself, establish that the impairment caused the collision. A driver with a measurable blood alcohol concentration may have been struck by another vehicle under circumstances that gave no attentive driver time to react. A driver under the influence may have been operating within posted speed limits on the correct side of the road when another party’s actions produced the collision. The reconstruction expert’s responsibility is to determine what the physical evidence shows about causation — independent of the impairment evidence — and to state plainly whether the driver’s operation of the vehicle contributed to the outcome and, where the impairment evidence is established, whether impairment may have been a contributing factor.

This analysis is done through the standard reconstruction methodology: pre-crash speed from event data recorder evidence, braking and steering input from EDR data and roadway evidence, sight-line analysis, perception-response time evaluation, and comparison of the driver’s documented response with published human factors data for an attentive driver. If the reconstruction shows that a sober driver operating under identical conditions would have had sufficient time and distance to perceive and avoid the hazard — and that the subject driver did not take effective action — the reconstruction shows that the collision was avoidable with a typical response. Whether that failure is attributable to impairment is ultimately for the trier of fact, weighing the impairment evidence alongside the reconstruction.

Conversely, if the reconstruction shows that the collision would have occurred regardless of whether the driver was impaired — because the hazard appeared with insufficient time for a reasonably attentive driver to respond — the expert’s analysis supports a different conclusion about causation. This kind of finding can be significant in a defense case.

Event Data Recorder Evidence in DUI Cases

Event data recorder information is often among the most important evidence in a DUI or vehicular homicide reconstruction. When the collision triggered a recorded event, the EDR data shows pre-crash indicated speed, accelerator position, and brake switch status, the driver’s belt buckle status, and the change in velocity during the collision, and many vehicles also record steering input. In cases where the central question is whether the driver was operating at excessive speed, failed to brake, or failed to take evasive action, EDR data provides a recorded account, independent of witness memory, that can be checked against the physical evidence.

For the prosecution, EDR evidence can show facts that bear on the charge: speed relative to conditions, the absence or timing of braking, and whether evasive steering was attempted. For the defense, the same data can show that the driver did brake, did attempt evasive action, or was at or near the posted limit — facts that may undermine the prosecution’s causation theory.

Event data recorder data has limits that belong in the report next to the numbers. 49 CFR Part 563 does not require a vehicle to carry a recorder — it specifies what one must capture if the manufacturer fits it. The pre-crash buffer is short and sampled at intervals, not continuous, and it is timed from time zero — the point at which the module’s algorithm woke or first detected a small change in velocity — rather than from first contact. Recorded speed generally comes from wheel or powertrain sensors, so tire size, lockup, wheel spin, and ABS intervention can move it off true ground speed. The recorded change in velocity is captured over a window that can close before a long or multiple impact is over, in many vehicles only the longitudinal component is recorded, and the federal specification allows ten percent on it. Records from events where nothing deployed can be overwritten, and no single retrieval tool covers every vehicle. None of that makes the data less useful — it defines what the record can support, and what has to be established another way.

EDR extraction in criminal cases requires strict attention to chain of custody. The vehicle is documented in its post-collision condition, the download is performed with current software, and the retrieved data is preserved in its original form with appropriate documentation. Gerald McDevitt uses the Bosch Crash Data Retrieval tool with a documented chain of custody.

Driver Perception-Response Analysis

Perception-response analysis is central to most DUI and vehicular homicide reconstructions. The question is typically whether the driver had sufficient time and distance to perceive the hazard and take effective action — and whether the driver’s actual response was consistent with that of a reasonably attentive driver.

The published research on perception-response time reports ranges of values for various hazard scenarios, ambient conditions, and driver populations. A reconstruction expert translates these values into the specific facts of the case: given the distance at which the hazard became reasonably detectable, the vehicle’s closing speed, and the time required for perception and response, could a reasonably attentive driver have avoided the collision?

The published research also addresses the effects of alcohol and drug impairment on perception-response time, attention, and judgment. Drawing on his law enforcement training and experience in impaired-driving investigation and on that published research, Gerald McDevitt can address whether impairment at or above the 0.08 legal limit for most drivers may have been a contributing factor. Where a case calls for a quantified analysis of how impairment modified perception-response, a toxicologist or human factors expert can address that question, and the reconstruction findings can be compared with that analysis.

Physical Evidence Specific to DUI Cases

Beyond the standard reconstruction evidence, DUI and vehicular homicide cases often involve physical evidence with particular significance. Lane departure patterns — whether the subject vehicle was drifting within its lane, crossing into oncoming traffic, or traveling on the shoulder before impact — can support inferences about driver attention and control. The absence of tire marks has to be read carefully, because vehicles with anti-lock brakes often leave little or no marking even under hard braking; EDR brake-switch and ABS-activity data, read with the recorded speed change, are the better indicator. Pre-impact yaw or loss-of-control marks in circumstances that should not have produced them indicate a loss of control that needs explaining — impairment is one possible explanation, alongside evasive steering, road conditions, or a vehicle defect.

Secondary-impact evidence — where a vehicle strikes a fixed object after the initial collision without any apparent attempt at control — has to be interpreted cautiously, because the first impact itself may have compromised the driver’s ability to control the vehicle. Rollover analysis in single-vehicle DUI cases often turns on whether the vehicle’s speed and input history support a theory of driver-induced loss of control versus an unavoidable environmental factor.

Scene documentation in these cases needs to be comprehensive, because both sides will test it. Gerald McDevitt uses the documentation tools each case calls for — Emlid Reach RS3 GNSS, a Sokkia reflectorless total station, Autel EVO II Pro RTK V3 drone aerial photogrammetry, Recon3D handheld LiDAR scanning, the Bosch Crash Data Retrieval tool, and the Racelogic VBOX GNSS data logger for supplementary testing — to build the record each case requires.

Working with Prosecution and Defense

Gerald McDevitt works both sides of DUI and vehicular homicide cases, and the methodology is identical regardless of the retaining party. Prosecutors retain him to determine what the physical evidence shows about causation. Defense counsel retain him to test the state’s reconstruction against the physical evidence, identify alternative explanations, and evaluate whether the prosecution’s theory survives scientific scrutiny.

The dual-role experience itself matters. An expert who works only prosecution cases is vulnerable to the claim that his analysis is adversarially shaped toward the state’s theory. An expert who works only defense cases is vulnerable to the corresponding claim from the prosecution side. An expert who works both — and whose findings follow the evidence rather than the retention — is better placed to answer that challenge. Under cross-examination, the question “isn’t it true you always find for the prosecution?” — or the defense equivalent — is answered by the record of the work itself.

What the Expert Can and Cannot Determine

Depending on the evidence available, a DUI or vehicular homicide reconstruction can address pre-crash vehicle speed, driver inputs, braking and evasive action, perception-response timing relative to published values, and whether the physical conditions allowed a reasonably attentive driver to perceive and respond effectively to the hazard. Where the impairment evidence is established, the expert can also address whether impairment may have been a contributing factor.

A reconstruction expert does not determine a driver’s blood alcohol concentration — that rests on toxicology and the other impairment evidence. The expert cannot determine the intent or conscious decision-making of the driver. And the expert cannot cross the line from analyzing driver conduct to pronouncing on criminal responsibility. The reconstruction provides the physical and scientific framework; the trier of fact applies that framework to the question of guilt.