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Road markings play an important role in road safety and optimizing the use of road space. They help provide information that cannot be easily conveyed using mounted signs.1 Moreover, signage applied directly to the road surface offers a form of continuous messaging which can be observed when a verge-mounted sign is concealed.
The effectiveness of road markings in improving the safety of road users relies on the clear visibility of markings. This becomes particularly vital when there is low light, such as during fog, rain, or night time. As the visibility of road markings is a critical factor in ensuring traffic safety, rigorous performance requirements have been introduced to guarantee the efficacy of road markings, which have to be checked and maintained regularly. European Reflectivity standards (European Standard EN 1436) specify the minimum levels of daytime and night-time visibility and also color and skid resistance.3
Road markings are available in a variety of formats, including spray, thermoplastic, screed, ribline, and extrusion, but all have to fulfill stringent visibility requirements.
Improving the Visibility of Road Markings
Recent collaborative European research has established that the minimum distance at which road markings should be visible to drivers must be equivalent to two seconds of travel time5. Several factors decide the distance at which a road marking is visible.4,5 Most of them are related to the driver, for example, the driver’s vision, headlight strength, car cleanliness, or are unavoidable, for example, rain or glare from oncoming vehicles. However, the composition of the road marking can be designed to optimize its visibility in a variety of conditions. For instance, the bright color of road markings is maintained by using titanium dioxide pigment, and the accumulation of dirt on markings can be prevented by adding crystallized titanium dioxide. Such accumulation of dirt on markings is likely to reduce their visibility.
The majority of the light emitted by the headlights, which hit the surface of the road, is either reflected forwards or absorbed by the road surface itself with just a fraction of the light reflected back towards the driver's eyes. Retroreflection is referred to as the reflecting of light back in the direction of the light source.5,6 As the coefficient of retroreflected luminance increases, the contrast between the road surface and the road marking also increases. When the light from the headlights which a road marking reflects back to the driver is more, the visibility of the road marking will also be more, particularly in bad weather and at night.
Additives such as titanium dioxide and others do not create retroreflection to improve the luminescence of road markings. On the contrary, retroreflection is increased by the addition of glass beads, thereby enhancing the night-time visibility of road markings. After the headlight beam enters the glass bead, it hits the pigmented road marking and is reflected back towards the driver of the car. As a result, the road marking appears to light up and hence the visibility of the road marking is considerably increased. Road markings, which include high-performance glass beads, are five times brighter than road markings that do not.
The level of retroreflection attained by glass beads is decided by the quality of the glass and the size of the beads. The 30 meter geometry is used to determine the level of retroreflectivity. This is the amount of reflected luminescence at a driver height of 1.2 m, an illumination distance of 30 m, and a headlamp height of 0.65 m.7 It is recommended to have a minimum retroreflectivity of 120 mcd/m2/ lux on a dry surface.
Glass Beads - Road Marking Enhancement
Usually, the glass beads used in road markings have a refractive index ranging between 1.5 and 1.9. They are developed in a wide range of sizes from 100 to 1500 microns in diameter and with different degrees of roundness. During production, glass beads can be combined into the road marking material (intermix beads); can be added when the road marking is applied (injection beads); or can be applied to the surface of newly applied road markings before they have set (drop-on beads).
It is important that the beads are embedded by at least 50% of their diameter to make sure that they do not become displaced. However, the level of retroreflectivity is reduced upon increasing the degree of bead embedment; therefore it is necessary to achieve an effective balance. It is expected that some of the beads will become covered with the marking material but this will soon be removed by passing traffic.
In addition, the quality of the retroreflection produced by the glass beads relies on the roundness and size of the beads, the viscosity of the road marking material, and the amount of beads added to the road marking. The larger beads with smoother, more round surfaces enable the highest retroreflective performance. An effective distribution level of glass beads is 400‑600 grams per square meter of road marking.
Mo-Sci Corporation, a global leader in high-quality precision glass technology, manufactures glass spheres for a variety of applications.8 Mo-Sci provides high-quality glass, which can be tailored to meet the specific requirements of projects. The company manufactures glass beads that are suitable for improving the visibility of road markings to tight specifications that ensure optimum reflectivity.
Road markings are an indispensable safety feature. Glass beads considerably increase the reflectivity of paints on the road, which in turn, considerably enhances their visibility and thus improves the safety of both drivers and pedestrians.
The only road marking additive that causes retroreflection is glass beads, which reflect more of the headlight beam back to the driver. As a result, glass beads make road markings to appear five times brighter at night time when compared to road markings without 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
This information has been sourced, reviewed and adapted from materials provided by Mo-Sci Corp.
For more information on this source, please visit Mo-Sci Corp.