Fast. Court-Ready. Repeatable

On-Scene Drag Factor / Friction Coefficient

Backed by peer-reviewed methods and trusted by police agencies worldwide, the VBOX Roadside Kit measures drag factor (f), position, speed and distance at the scene, with court-ready outputs.

Why measure drag factor on scene?

Serious crashes demand fast, defensible evidence.

The VBOX Roadside Kit lets officers run quick, controlled brake tests on the same surface and conditions as the incident to record an accurate drag factor (f) – the vehicle’s average deceleration expressed in g – with minimal setup.

One-touch operation, clear on-screen results, and standard exports keep the workflow simple, cut scene time, and preserve perishable evidence – so roads reopen sooner and documentation stands up in court.

The kit also measures position and average speed over a set distance, and records lateral and longitudinal G-force, delivering clean, usable case files and simplifying scene documentation and forensic mapping.

Why choose VBOX Roadside Kit?

Preserve perishable evidence

Capture a scene-representative deceleration coefficient (drag factor) along with time, position, distance, and motion before conditions change.

Quick to deploy

Mount, power on, run a test. No survey kit or complex setup required.

Simple touchscreen workflow

One-touch operation with minimal training; clear on-screen results. No PC or smartphone required.

Courtroom ready outputs

Export standard CSV/TXT reports suitable for case files and disclosure.

ISO/IEC 17025:2017 calibrated

Equipment supplied with traceable certificates and documentation.

Compatible and practical

Works with in-vehicle power or battery; includes external antenna and suction mount.

Budget-friendly

Priced to equip multiple patrol vehicles. Free VBOX Test Suite and no licence fees keep costs low.

Life-time Product Support

High-quality, free support for the life of your product

What is included in the Roadside Kit?

Performance Box Touch with pre-installed Drag Factor App
In-vehicle power supply and battery pack with approximately 6 hours runtime
Suction mount
Rugged carry case
Quick-start police setup guide
Built-in GNSS antenna
Additional external magnetic GNSS antenna if needed
Free VBOX Test Suite analysis software

Drag Factor App Modes

Measure drag factor from controlled braking/skid tests, validate average speed over a known distance, and record longitudinal and lateral G-force.

Drag Factor

Efficiently calculate deceleration and roadway drag factor (f) from a braking test.

Automatically logs time, distance, peak G, and drag factor
Results export in .CSV or .TXT with easy analysis in VBOX Test Suite
Optimised reporting for court exhibits and internal reporting

Speed and Distance

Measure average speed over a defined distance using start and stop buttons on screen. Ideal for validating speed over long or short distances, such as checking against CCTV or speed cameras.

One-touch operation while driving
Speed and distance displayed in real-time on the touchscreen
Event markers embedded in the results for post-analysis

G-Ball

Visualise lateral and longitudinal acceleration in real time to document and explain vehicle motion. Ideal for controlled test runs, officer training, and forensic scene reconstruction.

Displays live lateral and longitudinal G-force with an intuitive G-Ball interface
Captures peak lateral, acceleration, and deceleration values for post-test reference
Helps quantify peak forces experienced during a manoeuvre, supporting accurate crash analysis

Who uses the VBOX Roadside Kit?

A dedicated on-scene investigation tool for police and collision units, letting officers capture brake tests and scene data on the same surface and conditions shortly after an incident.

Accident & Collision Investigation Units

Specialist collision teams, traffic investigators, CIUs.

Highway Patrol & Police Forces

State, county and municipal patrol officers.

Police Training Academies

Driver training and driving standards units.

Ready to equip your team?
Talk to us about deploying the VBOX Roadside Kit in your department.

Get A Quote

Need advanced reconstruction tools?
For IMU-aided testing and multi-vehicle analysis, see our higher-end kits.

Advanced Accident Reconstruction

FAQs

What is drag factor in accident reconstruction?

Drag factor (f) = average deceleration of the whole vehicle

It’s obtained from a braking/skid test or deceleration measurement and reflects the combined effects of tyres, brakes/ABS, brake balance, rolling/aero resistance, vehicle mass distribution, contaminants, and grade (slope).

Why measure the drag factor at the scene?

An accurate drag factor (f) is essential when determining how fast a vehicle was travelling prior to a traffic accident. Measuring drag factor on the crash roadway, and under the same surface and conditions where safe and feasible, provides a scene-representative deceleration coefficient that can be used directly in speed-from-skids and braking distance calculations for accident reconstruction.

Measuring the drag factor (f) at the scene is critical for reliable reconstruction because it captures the combined effects of the test vehicle, tyres, brakes/ABS, and surface condition. This improves the defensibility of the analysis in traffic-accident and forensic contexts. When using f in reconstruction, document conditions and apply grade (slope) correction where relevant so the derived speed reflects level-road assumptions.

If you have a measured drag factor (f) for the scene, you generally do not need the coefficient of friction (μ) to compute speed for that specific braking event. However, the coefficient of friction (μ) – also called the friction coefficient – remains relevant when you:

Can’t test on scene and must rely on literature values or surface databases for a plausible friction coefficient (μ).
Validate reasonableness of your measured f (e.g., f should not exceed a plausible μ for that surface and condition).
Simulate (e.g., PC-Crash) or run sensitivity analyses where you separate the surface limit (μ) from vehicle-system effects captured in f.
Analyse non-braking manoeuvres that depend on lateral friction (e.g., critical speed yaw uses lateral μ, not your braking f).
Extrapolate to other vehicles/tyres or different environmental conditions (temperature, contamination, wet/dry), where μ helps frame what is physically reasonable.

Is drag factor the same as the coefficient of friction?

No. In U.S. law-enforcement and accident-reconstruction practice, “drag factor” and “coefficient of friction” are often used interchangeably, but they are not technically the same thing.

Coefficient of friction (μ): deceleration limit for a sliding tyre–surface pair.
Drag factor (f): average deceleration of the whole vehicle during the test event.

They are equal only when all wheels are locked and sliding on a level surface with negligible other resistances. Otherwise, f is typically ≤ μ due to ABS cycling, brake balance, tyre condition, weight transfer, contaminants, grade, aero/rolling resistance, slope, etc.

Example (same road, μ = 0.80):

Car A (new tyres, good brakes) skids with:
Deceleration = 0.80 g
f = 0.80

Car B (worn tyres, weak rear brakes) skids with:
Deceleration = 0.63 g
f = 0.63

Coefficient of friction (μ) is the same. Drag factor (f) differs by vehicle/setup.