The FMCSA BASIC score is a measure of the safety performance of a motor carrier. While the exact methodology of arriving at the score is not published by the FMCSA, this article will help you understand the components that impact it.

Violation Severity
Roadside inspection violations are given a severity weight (from 1-10) based on how those violations correlate to crash occurrences and consequences. For example, if under-inflated tires lead to more crashes than missed body inspections, it will be assigned a higher severity weight. See appendix A of the SMS Methodology for details. Additional severity weight is applied to violations that result in a driver or vehicle being ordered out of service.
Adjudicated Citations
Citations issued by a state that are associated with a violation noted in the above section (Violation Severity) may be subsequently adjudicated. If a violation ends in a dismissal or not guilty judgement, the violation is removed; if it results in a different charge, the severity weight is set to 1 and is not subject to OOS weight.
Crash Severity
Similar to inspection violations, crashes are also assigned severity weights based on their impact. Crashes involving injuries, fatalities, or the release of hazardous materials are given higher weights. If a crash is deemed not preventable (see the next section), it will excluded from the the score.
Not Preventable Crashes
The Crash Preventability Determination Program allows motor carriers to submit evidence that a crash was not preventable, so that it can be excluded from the safety score.
Time Weights
As time progresses, on-road safety events are given a lower time weight. An under-inflated tire violation from 12 months ago has a lower time weight than the same violation from 2 months ago. Safety events older than 24 months are excluded from the safety score.
The FMCSA normalizes your BASIC score so that carriers of varying sizes can be adjudicated on the same scale.
To ensure motor carriers with fundamentally different types of vehicles/operations are not compared to each other, they are segmented into two groups:

  1. Combination - trucks/motor coach buses constitute 70% or more of the total power unit types in a fleet
  2. Straight - trucks/other vehicles constitute more than 30% of the total power unit types in a fleet
Safety Event Groups
Motor carriers are also segmented based on the number of safety events in which they've been involved. These groupings ensure that the score accounts for variability in samples sizes and levels of exposure. In essence, a motor carrier with 1 violation out of a sample size of 1, won't be treated the same as a carrier with 1 violation out of a sample size of 50.
Data Sufficiency
Sample size is taken into account before any adverse action is taken by FMCSA. The data sufficiency standard ensures that there are enough inspections or safety events for measurements to be meaningful. 
Percentile Rank
On-road measures are used to assign a percentile ranking to each category (BASIC) for motor carriers. Higher percentiles are indicative of worse performance. Carriers above certain percentile thresholds that also meet the data sufficiency standards will be identified for potential FMCSA interventions.
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