Sampling Procedure for Performance-Based Road Maintenance Evaluations

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Sponsored By Virginia Department of
Transportation (VDOT)
Sampling Procedure for Performance-Based Road
Maintenance Evaluations
by Jesús M. de la Garza (chema_at_vt.edu), Juan C.
Piñero (rpinero_at_vt.edu), and Mehmet Egemen Ozbek
(meozbek_at_vt.edu), Virginia Tech
Sample Size Determination
Characteristics of Performance-Based Road
Maintenance Evaluations
Since 1990s, countries around the world have
been moving towards performance-based road
maintenance
Population and Sample Units
Performance Targets

• Population is defined by small road segments of a
  specific length (sample units)
• Sample units are 0.1 mile-long
• Sample units are considered independently on each
  direction (e.g., North and South)
• In a portion of a road that is 10 miles long the
  total population size will be 200 sample units
  (10 miles x 2 directions x 10 segments)
• Sample units have different asset items
• Divide the population in groups exposed to
  similar conditions, such as geographical
  location, weather variation, urban and rural
  settings, traffic volumes, and/or type of asset
  items. (Stratification)

• Acceptable quality levels
• Defined by the agency (i.e. state DOT)
• Must be considered in the development of the
  sampling procedure
• Different for each asset item
• 90 of pipes in a population should be in good
  condition ( i.e. meet specific performance
  criteria)
• 95 of traffic signs in a population should be
  in good condition ( i.e. meet specific
  performance criteria)
• The variability in performance targets results in
  a significant difference when determining the
  minimum number of samples required for each asset
  item

Performance Evaluation
Budget and Time Limitations
The sampling methodology to collect the
information needed
The portion of the population to be inspected
The frequency at which the inspections should be
conducted
The result obtained ( ) for each asset item at
the sample level can be generalized to the whole
population with the following confidence interval
at the relevant confidence level
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1. Stratify the population
2. Define the sample units
3. Identify the asset items in each sample unit
4. Create the database with the sample units
   containing each asset item
5. Define the values of parameters to be used in
   sample size formulas
6. Compute the required sample size for each asset
   item
7. Perform the random selection of sample units

• Approach 1 Keeping the Desired Precision Rate
  (e) Constant over Multiple Samplings
• Findings are always in the desired precision
• May result in substantial changes in the sample
  size from one evaluation to another
• Approach 2 Stating the Desired Precision Rate
  (e) Relative to the Populations Coefficient
  of Variation over Multiple Samplings
• Sample size does not change significantly
• The precision of the findings may be at stake
• Approach 3 Keeping the Sample Size Constant over
  Multiple Samplings
• Sample size is constant from one evaluation to
  another
• The precision of the findings stay in an
  acceptable range

• The developed procedure is effective
• Ensures that the findings from the field
  inspections will be reliable and representative
  with high confidence of the actual condition of
  the asset items in the entire population
• The developed procedure is efficient
• Envisions visiting a minimal number of sample
  units with many asset items as opposed to a large
  number of units with a few asset items
• The developed procedure is easy to implement
• Approaches are presented in a stage-by-stage and
  step-by-step manner in an effort to assist the
  interested transportation agencies to easily
  comprehend the concepts and implement the
  developed procedure
• The developed procedure has been implemented
  since 2002 to assist the Virginia Department of
  Transportation to monitor the performance of its
  contractors
• These are long term contracts which necessitated
  the development of the three alternative
  approaches of sampling as the sampling needs to
  be performed not just once, but multiple times
  over the course of the contracts duration

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