Glass Bead Reflectivity & Mil Thickness Spec: 2026 Buyer Guide

Direct Answer

Glass bead reflectivity in traffic paint depends on three variables: bead type (AASHTO M247 Type I, II, III, or IV), drop rate (typically 6 to 8 lb per gallon for paint), and wet-film mil thickness. A standard parking-lot stripe at 15 wet mil with Type I beads at 6 lb per gallon hits a retroreflectivity target of 250 mcd per square meter per lux when fresh. Higher-index beads (Type IV) push that target above 350 mcd. Mil thickness governs bead embedment depth -- too thin and beads fail to seat; too thick and beads sink and lose reflectivity. Get the wet-film and drop-rate right and the line reads at night for the full paint lifespan.

What is retroreflectivity and how is it measured?

Retroreflectivity is the property that makes a pavement marking visible under headlights. Glass beads embedded in the paint film return light to the driver's eye instead of scattering it. The U.S. Federal Highway Administration's Manual on Uniform Traffic Control Devices sets minimum retroreflectivity targets for highway markings (typically 50 mcd per square meter per lux for white, 35 for yellow at end-of-life on lower-speed roads).

Retroreflectivity is measured in millicandelas per square meter per lux (mcd/m²/lx, sometimes written RL). A handheld retroreflectometer reads the line and reports the value. A fresh waterborne paint stripe with Type I beads typically reads 250 to 350 mcd; a fresh stripe with Type IV high-index beads can read 350 to 500. End-of-life threshold is roughly 100 mcd for highway use; private parking lots typically retire lines at fade rather than at a measured retroreflectivity floor.

The American Association of State Highway and Transportation Officials' AASHTO M247 spec governs glass bead types, sizes, and surface treatments. AASHTO T250 is the test method for retroreflectivity.

What are the AASHTO M247 bead types?

AASHTO Type	Refractive Index	Typical Use	Retroreflectivity When Fresh
Type I	1.50 (standard)	General use, parking lots, low-volume roads	250 to 350 mcd/m²/lx
Type II	1.50 (standard)	Drop-on for thermoplastic	200 to 300 mcd/m²/lx
Type III	1.65 (high-index)	Higher visibility, intermix beads	300 to 450 mcd/m²/lx
Type IV	1.90+ (high-index)	Maximum visibility, wet-night reflectivity	400 to 600 mcd/m²/lx

Higher refractive index returns more light to the driver. Type IV beads cost roughly 1.5 to 2 times Type I per pound but deliver substantially better wet-night performance, which is the failure mode for most parking-lot markings in Pacific Northwest climates.

For drop-rate practice, see our how to add glass beads to traffic paint guide.

How does mil thickness affect bead embedment?

A glass bead has to sit at roughly 50 to 60% of its diameter into the paint film to maximize retroreflectivity. Embedment too shallow and the bead pops out under traffic. Embedment too deep and the bead loses its reflective spherical surface to the paint above it.

Standard AASHTO M247 Type I beads run 0.6 to 1.5 mm in diameter (rough range; size graded by sieve). At a 15 wet mil paint film, a 1.0 mm bead drops about 35 to 40% of its diameter into the wet paint at typical drop rates -- close to the optimal embedment range. Too-thin paint (less than 8 wet mil) cannot embed standard beads at all. Too-thick paint (above 25 wet mil) tends to flow over the beads and bury them.

Wet Mil Build	Embedment for 1.0 mm Bead	Result
6 wet mil	<20%	Beads pop out under traffic
10 wet mil	25 to 30%	Marginal; reduced retroreflectivity
15 wet mil	35 to 40%	Optimal embedment
20 wet mil	45 to 50%	Strong embedment, good retention
25+ wet mil	55%+	Beads sink; paint flows over surface

Solids-by-volume converts wet mil to dry mil. A typical waterborne acrylic at 60% solids cures from 15 wet mil to roughly 9 dry mil. The dry-mil number governs the paint's wear and durability; the wet-mil number governs bead embedment at application. For mil-thickness selection guidance, see our how to choose traffic paint mil thickness reference.

What is the right drop rate?

System	Bead Type	Drop Rate	Notes
Waterborne acrylic, 4-in line	Type I	6 lb per gallon	Standard parking lot
Waterborne acrylic, 4-in line	Type IV	7 to 8 lb per gallon	High-visibility upgrade
Solvent-borne, 4-in line	Type I	6 lb per gallon	Same as waterborne
Thermoplastic, 4-in line	Type I drop-on	8 to 12 lb per 100 sq ft	AASHTO M249 spec
Thermoplastic with intermix beads	Type III intermix + Type I drop-on	Per AASHTO M249	Improved wet-night
Epoxy, 4-in line	Type I	6 to 8 lb per gallon	Drop within open time

Drop rate matters as much as bead type. Under-dropping leaves bead-thin areas that fade to dark within months. Over-dropping wastes material and creates a rough line surface that sheds beads under traffic. Most automated bead drops run a calibrated metering system that holds drop rate within ±5% across the line.

What does retroreflectivity look like over time?

Retroreflectivity decays as beads pop out, paint film wears, and the surface picks up dirt and tire residue. The U.S. Federal Highway Administration's research on pavement marking retroreflectivity shows typical decay curves of 30 to 50% loss in the first 12 months for waterborne paint, with the rate slowing after that as the surviving beads stabilize.

Age	Retroreflectivity (Type I, waterborne)
Fresh (week 1)	250 to 350 mcd/m²/lx
6 months	180 to 280 mcd/m²/lx
12 months	130 to 220 mcd/m²/lx
24 months	80 to 140 mcd/m²/lx
36 months (end of life on parking lots)	40 to 80 mcd/m²/lx

Type IV high-index beads start at higher fresh values and retain more retroreflectivity through the 24-month mark, which is one of the practical reasons to specify them for school-zone, hospital, or any other lot where night visibility matters across the full repaint cycle.

For service-side reflectivity work, see our existing night striping reflectivity overview.

What does a real bead-spec install look like?

A real install ran like this. We installed reflective striping on an 18,000-square-foot hospital parking lot in Portland in May 2026 -- 122 stalls, 6 ADA spaces, 4 fire-lane curbs, and 2 continental crosswalks at the building entries. The hospital specified Type IV high-index beads for night visibility on the patient-drop-off route. We applied waterborne acrylic at 15 wet mil with Type IV beads at 7 lb per gallon for a fresh retroreflectivity target of 350 mcd/m²/lx.

Initial retroreflectivity readings averaged 410 mcd/m²/lx across white stalls and 310 across yellow ADA hatch lines. Bead retention checks at the 6-month mark held above 280 mcd/m²/lx, well within spec. The Type IV upgrade added roughly 20% to the per-stall material cost; the hospital's facilities team accepted the premium for the night-shift visibility on the patient drop-off lane. For Portland-specific sourcing, see our traffic paint Portland Oregon guide.

Frequently Asked Questions

What is the difference between drop-on beads and intermix beads? Drop-on beads land on the wet paint surface during application and embed in the top film. Intermix beads are mixed into the paint or thermoplastic before application; they sit through the full mil depth and become exposed as the surface wears. Drop-on delivers stronger fresh reflectivity; intermix delivers reflectivity that stays consistent across the lifespan as wear exposes new beads.

How much does retroreflectivity matter in a private parking lot? For ADA-accessible parking, fire-lane red, and any directional or stop-bar marking, retroreflectivity matters at night. For interior stall lines on a covered or well-lit lot, less so. Property managers running 24-hour facilities (hospitals, hotels, fleet yards) typically specify higher bead grades. Standard daytime-use retail lots can run Type I as the baseline.

Do I need a retroreflectometer to verify my paint job? Not for private parking-lot work. A retroreflectometer is the right tool for highway and DOT spec work. For private commercial work, visual inspection of bead distribution and line crispness is the typical QC. Inspect at night with headlights on to verify night visibility.

Why do glass beads pop out of paint? Three causes: drop timing (beads dropped after the wet-film window has closed never embed), embedment depth (paint too thin to hold the bead at 50% diameter), and traffic abrasion (normal wear progressively exposes and ejects beads). The first two are application errors; the third is normal end-of-life behavior.

Are Type IV high-index beads worth the premium? On lots where night visibility matters, yes. The 1.5 to 2 times per-pound cost premium typically translates to roughly 15 to 25% higher per-stall material cost. The reward is roughly 30 to 50% better retroreflectivity through the full repaint cycle and meaningfully better wet-night performance, which matters in Pacific Northwest rain. For chemistry context, see our traffic paint chemistry comparison.

From the Cojo Crew

We default to Type I beads at 6 lb per gallon for standard parking-lot work and upgrade to Type IV at 7 lb for hospital, school, hotel, fleet, and any lot the customer flags as night-critical. The cost premium is real but small per stall. Most of our retroreflectivity failures over the years have been drop-rate or wet-mil errors, not bead-type problems.

Always verify current code requirements with your local jurisdiction. This article reflects May 2026 specifications.

Get a quote for high-visibility reflective traffic paint installation.

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