Black box evidence in trucking.

Three kinds of electronic evidence

Modern commercial trucks have multiple electronic systems that record data relevant to crash analysis:

ECM — Engine Control Module

The truck's primary engine computer. Records and stores data continuously, including engine RPM, vehicle speed, brake application, throttle position, cruise control status, and fault codes. Most ECMs also store "hard brake" events (panic stops) and "fault events" with timestamped data.

EDR — Event Data Recorder

Similar to airplane "black boxes" — captures data in a rolling window around significant events (typically severe braking, airbag deployment, or rapid deceleration). Records the seconds before, during, and after a crash event in detail.

ELD — Electronic Logging Device

Required since December 2017 under the ELD Mandate (49 CFR Part 395 Subpart B). Records driver duty status (driving, on-duty not driving, sleeper berth, off-duty), location, engine hours, and vehicle miles. Replaces paper logbooks for Hours of Service compliance.

Each of these systems can be downloaded after a crash — but only if the right equipment and protocols are used, and only if the data hasn't been overwritten yet.

What ECM data reveals

A properly downloaded ECM gives the accident investigator a detailed picture of what the truck was doing in the moments leading up to and during a crash. Common data elements:

• Vehicle speed — Often available at 1-second intervals over a window of 60-300 seconds before the event
• Engine RPM — Helps determine whether the driver was accelerating, decelerating, or coasting
• Brake application — Yes/no flag and (in some systems) pressure
• Throttle position — Percentage of accelerator pedal application
• Cruise control status — Was cruise engaged?
• Clutch application (manual transmissions)
• Gear and transmission status
• Hard brake events — Timestamped events where braking exceeded threshold
• Fault codes — Mechanical or electronic failures detected by the ECM

The data is typically pulled by connecting a laptop to the truck's diagnostic port using manufacturer-specific software (Detroit Diesel's DDDL, Cummins Insite, Caterpillar ET, Volvo VCADS, etc.). Different engine manufacturers store different data sets, so the analyst needs to know what to look for in each system.

What ELD data reveals

ELDs were mandated to enforce Hours of Service compliance — and they generate a separate, highly detailed record set that's invaluable in trucking litigation:

• Duty status changes — When the driver went on-duty, started driving, went off-duty, entered sleeper berth
• Location data — GPS coordinates at duty status changes and at one-hour intervals during driving
• Engine hours and vehicle miles — Engine on/off time and accumulated miles
• Unidentified driving — Time the truck was driven without a driver logged in (often a sign of HOS evasion)
• Edits to log entries — When and by whom logs were modified
• Driver and motor carrier identifying information

ELD data must be retained by the motor carrier for 6 months under § 395.8(k). Compare ELD data to:

• Fuel receipts (showing driver was at a fuel stop during claimed "off duty" time)
• Toll records and ELS readers (showing actual driving location)
• Bills of lading (showing pickup/delivery times)
• Dispatch records

Discrepancies between these sources and the ELD record can reveal Hours of Service violations even when the ELD log itself looks compliant — meaning the driver edited logs or operated under unidentified driving.

The 30-90 day overwrite problem

ECM and EDR memory is limited. The systems use rolling buffers — recording continuously and overwriting older data as the buffer fills. Different systems have different retention windows:

• ECM hard-brake events — Typically 1-5 events retained; oldest gets overwritten as new events occur
• EDR pre-event data — Typically 30 days to several months, depending on system and use
• ELD records — 6-month minimum retention by carrier (§ 395.8(k))
• Vehicle continuing in service — Continues generating new events that may overwrite the crash event

This creates a critical preservation problem: if the truck is repaired and put back in service, every hard brake by the next driver pushes the crash data closer to being overwritten. If no preservation step is taken, the data can be gone in weeks or months.

Preservation letters and what they should say

A proper preservation letter does more than say "preserve evidence." It identifies, specifically:

• The crash (date, location, vehicles involved, driver name)
• The categories of evidence to preserve (ECM data, ELD data, dashcam footage, telematics data, driver's logs, DVIRs, maintenance records)
• The specific federal regulations requiring retention of each category
• The retention period for each (so the carrier can't argue ambiguity)
• An explicit request to refrain from repairing, modifying, or operating the truck until the ECM is downloaded
• A demand for written confirmation that preservation measures have been implemented

If the preservation letter is ignored and evidence is destroyed, the basis for spoliation sanctions is established. Texas case law on spoliation includes Brookshire Bros. v. Aldridge, 438 S.W.3d 9 (Tex. 2014) — which set the framework for spoliation findings and remedies. Sanctions can include adverse inference instructions, exclusion of defenses, and (in extreme cases) default judgment.

How ECM evidence is used at trial

Once downloaded, ECM data becomes one of the most powerful pieces of evidence in a truck crash case. Common uses:

Contradicting driver testimony

Driver claims he was going 60 mph — ECM shows 72. Driver claims he braked immediately — ECM shows no brake application until 1.2 seconds before impact. Driver claims he didn't see the plaintiff's vehicle — ECM shows no throttle reduction or steering input.

Establishing speed and reaction time

Accident reconstruction experts use ECM data as the input for technical analysis — speed at impact, available reaction time, distance traveled at given speeds. This anchors the entire case in objective data rather than disputed eyewitness testimony.

Proving fatigue or HOS violations

ECM engine-on hours combined with ELD records can show the driver had exceeded duty limits before the crash. Many cases reveal "ghost driving" — periods where the truck was operating but no driver was logged on the ELD.

Establishing mechanical defects

Fault codes preserved in ECM memory may reveal mechanical issues — brake system warnings, ABS failures, engine governor malfunctions — that contributed to the crash and the carrier knew about.

Other electronic evidence beyond ECM

Modern commercial trucks may have additional electronic systems that are equally valuable:

Dashcam systems

Many carriers now use forward-facing and/or driver-facing dashcam systems (Lytx DriveCam, SmartDrive, Samsara). These typically retain video for 30-180 days. Critical to identify whether a dashcam was present and preserve any footage.

Telematics platforms

Carriers commonly use telematics systems (Omnitracs, Trimble, ISAAC, Verizon Connect) that integrate ECM, ELD, GPS, and other data. These systems often retain richer data sets than the on-board ECM alone.

Mobile communication / dispatch systems

Records of communications between driver and dispatch — including pressure to make delivery times, route changes, and acknowledged equipment problems — can show carrier culpability beyond the driver's individual conduct.

Adaptive cruise / collision-warning system data

Many trucks now have advanced driver-assistance systems (ADAS) that record warning events — forward collision alerts, lane departure warnings, hard brake activations. This data can show the driver had pre-collision warnings and failed to respond.