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About Ra Readings

Retroreflectivity (Ra) is type of reflection that redirects light back to its source. Today’s traffic sign sheeting materials are enhanced with prismatic reflectors that produce Ra from vehicle headlights, making signs appear brighter and more visible to nighttime drivers.

In 2003, the Federal Highway Administration (FHWA) began establishing minimum Ra standards for certain types of traffic signs including:

  • Regulatory
  • Warning
  • Street name
  • Guide

Cartegraph is a very useful tool for helping organizations achieve and maintain FHWA Ra compliance. Use Sign inspection data captured and entered in Cartegraph to identify necessary sign repairs and replacements, and predict the future condition of inspected signs.

Here’s how:

  • Enter your Sign inspection values, select your Assessment method, and enter the Sign readings in the Legend Ra and Background Ra fields. A contrast ratio automatically calculates.
  • The contrast ratio divides the higher Ra value by the lower Ra value. For example, if the Legend Ra is 53 and the Background Ra is 230. The Contrast Ratio is 4.33, 230/53.
  • Because there are condition categories for Background Ra and Legend Ra, entering values directly into the inspection fields affects future performance prediction, calculates an average, and updates the inspection fields for Ra.

The FHWA recommends using two methods for conducting Ra assessments measured sign retroreflecivity and visual nighttime inspection:

Measured Sign Retroreflectivity

  • Performed with the help of retroreflectometer
  • Measurement done on each sign face
  • Result is produced by averaging four separate measurements

Visual Nighttime Inspection

  • Performed visually at night by a trained sign inspector
  • Assessment made from a vehicle to simulate driving conditions
  • Simple means of identifying nighttime visibility issues
  • Also useful for detecting damage, obstructions, poor placement, and more

The FHWA recommends combining these two methods for sign inspections. Use a retroreflectometer to get an accurate measurement of Ra factors, then visually inspect other attributes such as damage, obstructions, poor placement, and more.