Road markings make a vital contribution to road safety and optimizing use of road space and make it possible to provide information that it is not easy to convey using mounted signs.1 Furthermore, signage applied directly to the road surface provides a form of continuous messaging which can be seen when a verge-mounted sign may be obscured.
The effectiveness of road markings in enhancing the safety of road users depends on the markings being clearly visible. This becomes particularly important at times of low light, such as during night time, rain or fog. Since the visibility of road markings is a crucial factor in ensuring traffic safety, strict performance requirements have been introduced to ensure the efficacy of road markings, which must be checked and maintained on a regular basis. European reflectivity standards (European Standard EN 1436) stipulate the minimum levels of daytime and night-time visibility as well as color and skid resistance.3
Road markings come in a range of formats, including thermoplastic, screed, spray, extrusion and ribline, but all must meet strict visibility requirements.
Improving the Visibility of Road Markings
Recent collaborative European research has determined the minimum distance at which road markings should be visible to drivers to be equivalent to two seconds of travel time.5 The distance from which a road marking is visible depends on a variety of factors.4,5 Many of these are driver-related, e.g., the driver’s vision, car cleanliness, headlight strength; or unavoidable, e.g., glare from oncoming vehicles, rain. The composition of the road marking however can be designed to optimize its visibility in a range of conditions. For example, titanium dioxide pigment is used to maintain the bright color of road markings and crystallized titanium dioxide is added to prevent the build-up of dirt on markings, which would reduce their visibility.
Most of the light emitted by the headlights which hits the road surface is reflected forwards or absorbed by the road surface itself with only a fraction of the light reflected back towards the driver’s eyes. The reflecting of light back in the direction of the light source is known as retroreflection.5,6 The higher the coefficient of retroreflected luminance the greater the contrast between the road marking and the road surface. The more of the light from the headlights that a road marking reflects back to the driver, the more visible the road marking will be, especially at night and in bad weather.
Titanium dioxide and other such additives do not create retroreflection to enhance the luminescence of road markings. In contrast, the addition of glass beads increases retroreflection, whereby improving the night-time visibility of road markings. The headlight beam penetrates the glass bead, hits the pigmented road marking, and is reflected back towards the car driver. The road marking therefore appears to light up and so the visibility of the road marking is increased significantly. Road markings incorporating high performance glass beads are five times brighter than road markings that do not.
The extent of retroreflection achieved by glass beads depends on the size of the beads and the quality of the glass. The level of retroreflectivity is determined using the 30-metre geometry. This is the amount of reflected luminescence at an illumination distance of 30 m, a driver height of 1.2 m, and a headlamp height of 0.65 m.7 A minimum retroreflectivity of 120 mcd/m2/ lux on a dry surface is recommended.
Glass Beads – Road Marking Enhancement
Glass beads for use in road markings typically have a refractive index of between 1.5 and 1.9. They are produced in a variety of sizes ranging from 100 to 1500 microns in diameter and with varying degrees of roundness. Glass beads can be mixed into the road marking material during production (intermix beads), added as the road marking is applied (injection beads) or applied to the surface of newly applied road markings before they have set (drop-on beads).
The beads must be embedded by at least 50% of their diameter to ensure that they do not become dislodged. However, increasing the degree of bead embedment reduces the level of retroreflectivity, so an effective balance needs to be achieved. It is inevitable that some of the beads will become covered with the marking material but this will soon be rubbed off by passing traffic.
The quality of the retroreflection produced by the glass beads also depends on the size and roundness of the beads, the amount of beads added to the road marking and the viscosity of the road marking material. Highest retroreflective performance is achieved using the larger beads with smoother, more round surfaces. An effective distribution level of glass beads is 400 to 600 grams per square meter of road marking.
Mo-Sci Corporation produces glass spheres for a wide range of applications.8 Mo-Sci supplies high quality glass, which can be customized to meet specific project requirements. They produce glass beads suitable for increasing the visibility of road markings to tight specifications that guarantee optimum reflectivity.
Conclusion
Road markings are an essential safety feature. Glass beads significantly increase the reflectivity of paints on the road, which in turn significantly improves their visibility and consequently driver and pedestrian safety.
Glass beads are the only road marking additive that causes retroreflection, sending more of the headlight beam back to the driver. Consequently, glass beads make road markings at night time appear five times brighter than road markings that do not contain glass beads.
References & Further Reading
- Department of transport UK 2003. Traffic signs manual Chapter 5 Road markings. Available at https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/223667/traffic-signs-manual-chapter-05.pdf
- Charlton SG, et al. Using road markings as a continuous cue for speed choice. Accid Anal Prev. 2018;117:288?297. doi: 10.1016/j.aap.2018.04.029. Epub 2018 May 9.
- Highways Markings. A Guide to IS EN 1436 European Standard for Road Markings. Available at http://www.highwaymarkings.ie/documents/is_en_1436_1.pdf
- Owens Da, et al. Effects of age and illumination on night driving: a road test. Hum Factors 2007;49(6):1115?1131.
- The National Cooperative Highway Research Program. Chapter 3. Available at http://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_syn_306_22-37.pdf
- Stoudt MD, Vedam K. Retroreflection from spherical glass beads in highway pavement markings. 1: Specular reflection. Applied Optics 1978;17:1855?1858.
- Pike AM, et al. Evaluation of Retroreflectivity Measurement Techniques for Profiled and Rumble Stripe Pavement Markings. Transportation Research Record 2011. Paper 11-1293. Available at http://docs.trb.org/prp/11-1293.pdf
- Mo-Sci Corporation. Company website available at http://www.mo-sci.com