Surface friction is crucial for the safety of all road users at all locations. It is even more important for motorcycles than for two-track vehicles, since loss of grip may lead to instability, loss of balance and crashes. Riders must be aware of dangerous surface conditions, how to detect them and ride in a speed where he/she can maintain the ability of either stopping in advance or swerving around low-friction areas. However, poor friction cannot always be detected by visual scanning and there is no possibility of a correct estimation of a wet road’s friction coefficient with the eyes only.

The TRB’s National Cooperative Highway Research Program NCHRP Guide for Pavement Friction mentions several sources that indicate a direct relation between road surface frictions and crashes. A comprehensive evaluation of friction measurements and crash rates revealed that increasing pavement friction reduces crash rates significantly (55). This chapter describes common causes for loss of control due to poor friction and measures to minimize the crash risk.

6.1 ROAD MARKINGS, DRAIN COVERS, TRAM RAILS AND CATTLE GUARDS
Road markings, drain covers, tram rails and cattle guards tend to be more slippery than the rest of the road surface, especially when wet. For motorcyclists these areas can create problems, especially when the rider needs to brake and turn.

Possible solutions
There are several documents which presents solutions to the problems. The following comes from the Norwegian handbook for motorcycle safety, but the content is similar to the advice from Vicroads in the guide for Making Roads Motorcycle Friendly (33, 36).

• Select road marking material with best possible friction. This is especially important in areas where motorcyclists must brake and/or turn.
• Include a minimum friction requirement on road marking in EN 1436:2018 – Road marking materials – Road marking performance for road users and test methods.
• Placement of pedestrian crossing on a curve should be avoided. Motorcyclists may skid on the slippery markings and because the visibility to pedestrians and cyclists is poor.
• Avoid pavement markings in the lanes closer than ten meters from a stop or yield line which will give the motorcycle an adequate area for braking and turning at intersections.
• Avoid cattle guards around curves.
• Avoid drain covers on roads and streets, particularly on curves and intersections within carriageways. Crash risk of existing drain covers on roads should be minimized by installing them at the same level as the rest of the road, using anti-skid drain covers and repair broken covers as soon as possible.

6.2 URBAN AREAS
Solutions for increased friction to reduce the crash risk and other useful measures to improve motorcycle safety in urban areas can be found in the Transport for London’s Urban Motorcycle Design Handbook. The Handbook recommends that (35):

• Block paving and stone sett entry treatments are positioned away from areas where motorcyclists are required to turn. At intersections this can be achieved by locating ramps further into the side road.
• Surface materials are laid on a robust sub-base, with an appropriate flush edge detail provided at the transition point between the surface types.
• A regular inspection, maintenance and repair regime is employed to ensure carriageway defects likely to affect motorcycle stability are identified and repaired in time.
• As to the extent to which on-road markings are required, the Handbook recommends that:
  • The level and size of markings is proportional to the degree of potential hazard and consistent along the route (i.e., remove/do not provide unnecessary markings).
  • Consideration is given whether advanced warning and direction signs can be used to minimize the need for surface markings.
  • Where possible, position markings are positioned away from motorcycle steering, braking, and accelerating zones and suitably placed in advance of bends or junctions rather than within them.
  • Markings have a similar skid resistance to the surrounding road surface.
  • Future maintenance regimes – avoid repeated application of road marking material as this can form ridges that can cause deviation of a motorcycle.
  • The use of black paint to cover over markings that are no longer required is avoided as this can form a skid hazard.

6.3 LOSS OF FRICTION DUE TO GRAVEL FROM UNSEALED SHOULDERS AND ONCOMING GRAVEL ROADS
Gravel from unsealed shoulders and side roads is a hazard for all road users but especially motorcyclists. This problem, which can cause loss of friction, is most likely to happen in intersections and curves where gravel is dragged out on the road from roadsides and side roads by by cars and heavy traffic using the roadsides on narrow roads. Gravel roads are more common in areas with a low population density. Gravel and dirt from these roads can be dragged out on the asphalt roads by vehicles which can cause loss of friction as well as water from rain or snow. A study in 2017 commissioned by the Sveriges MotorCyklister (SMC) showed that loose gravel on asphalt leads to a reduction of friction from 0.8 to 0.35, which is comparable to winter conditions (57).

Possible solutions
Sealing the shoulder at higher-risk locations, such as on curves and parallel to side roads, will remove loose material from the roadside. This is a targeted infrastructure treatment that significantly reduce the risk of loose material being on the road surface at high surface friction demand locations where motorcycles are leaning through a curve. A risk-based approach could be adopted for shoulder sealing activities. Slovenia has used grid plates which turned out to be effective since gravel and sand were eliminated and cost-efficient with no need of maintenance work (36, 38, 39).

6.4 LOSS OF FRICTION DUE TO PATCH REPAIR AND BITUMEN
Crack sealing prevents water from entering the pavement causing other hazards such as potholes and tracks. Pavement damage can greatly affect the handling of a motorcycle which increases the risk of crashes, particularly on curves. Poor and/or wrongly performed patching can also result in an increased crash risk as the tar and bitumen repairs can become slippery in hot or wet conditions.

Possible solutions
Patching methods and materials used should give a lasting and safe result. When properly done, the patching material must be at the same level with the adjoining pavement and have a similar friction. The patching and repair of cracks done with bitumen is prohibited in several jurisdictions. If it is undertaken, however, it is essential that the material is treated with sand or other material in a correct manner to ensure the same friction as the adjoining pavement. In Slovenia the cracks need to be sprinkled with crushed stone of silicate origin (38). Loose material from the maintenance shall never remain at the site when it has opened to traffic or be removed as soon as possible. It is desired that they should be used as little as possible: this could be achieved by greater use of surface coatings.

6.5 LOSS OF FRICTION DUE TO GRAVEL FROM SURFACE DRESSING
Maintenance methods that include a gravel surface or loose gravel applied on top of asphalt and/or bitumen is known as surface dressing. It is an acceptable method for the short to medium term if carried out correctly. If not correctly undertaken, it can significantly increase the risk of loss of control for motorcycles. A preferred solution would be is to lead riders to other roads to avoid the area under road repair entirely. The time frame varies depending on the maintenance method. One common cause of poor friction are failures during and after road maintenance activities. This is the case in Sweden where gravel, mainly from road repair, is the most common cause for single vehicle as crashes according to a leading insurance company Svedea 2014-2021 (58).

A recent study based on seriously injured vulnerable road users registered in the Swedish accident database Strada for the years 2014–2019 shows that loose grit was the most common cause for crashes among moped riders and motorcyclists. The study found that 21% of the motorcyclists and 22% of the moped riders were seriously injured due to a crash caused by gravel (25).

Possible solutions
When surface dressing or similar techniques are used it is important that the road is mechanically swept after the gravel has been rolled and that the road is routinely cleaned until all surplus gravel is gone. This must be done within the decided time limits in the contract. Warning signs should alert drivers and riders to lose gravel conditions on the road.

In Norway, the National Motorcycle Strategy stipulates that the maximum length of a maintenance work with loose gravel on bitumen is five kilometres, then the road must be cleaned. This is a part of the national motorcycle strategy. Norway has also introduced new methods for maintenance where all gravel is removed immediately during the maintenance (36).

6.6 LOSS OF FRICTION DUE TO BLEEDING ASPHALT AND SEALINGS
Asphalt bleeding (also known as flushing) is where the asphalt binding agents liquify and rise to the surface of the tarmac. It appears as a glassy, highly reflective surface that looks like it is wet or oily. As the binding agents do not reabsorb, they can build up over time.

Bleeding asphalt is dangerous to riders and is often impossible for a rider to see. In wet conditions the friction can become comparable to winter with snow and ice. Asphalt bleeding can occur on roads with a new layer, but it can also appear during a heatwave or when the rain starts to fall after a long hot period. Asphalt bleeding may also be the result of poor manufacturing or installation. Another problem are longitudinal sealings between the lanes as they can be very slippery and have caused several severe and fatal crashes in Sweden. This is a part of the maintenance where one of two lanes on 2+1-roads are paved with new asphalt (59).

The Slovenian guidelines highlights that improper repair of cracks represents a danger for motorcyclists, especially if the repairs are done on a curve. This unexpected situation with bituminized lines of cracks offers significantly less grip than the road surface, especially in wet conditions (38).

Possible solutions:

• Friction tests should occur on a regular basis on roads with high loss-off-control motorcycle crash risk, based on motorcycle volumes, curve size and frequency and crash history.
• Use asphalt with good frictional properties (i.e., aggregates with low polishing values, high micro-texture subsurface draining asphalt or spray seals with large aggregate), especially when paving curvy road sections. It must be emphasized that the pavement shall have good frictional properties both when newly laid and after having been trafficked worn.
• When paving is terminated on a curve or at an intersection, altered friction or pavement joints can create problems for motorcyclists. Such resurfacing should, therefore, continue through bends.
• With structural or horizontal pavement milling, a significant frictional improvement can be achieved, especially on pavements with a wet surface. This can reduce the probability of motorcycle crashes also those involving other vehicles. Milling, however, should not be with a coarse pattern that inexperienced motorcyclists will become insecure and lose control.
• Operators of road networks must always be prepared for parts of the road network having frictional conditions that will be hazardous to motorcyclists. When an acute frictional problem is identified, for example in conjunction with diesel fuel spills or paving, immediate action is needed. It may be necessary to direct traffic to other roads or to stop motorcyclists manually. Signs must be put up and good and lasting improvement measures must be implemented as quickly as possible.
• The overflow of bitumen in sealings and cracks can be minimized with regulations and if there is an overflow, instant sanding must be done and removed later (60). Bituminizing of lines of cracks must immediately be followed by spreading of crushed quartz with fractions of 0.3-1.2 mm and not rock dust, since the latter does not ensure adequate grip (38).

6.7 LOSS OF FRICTION DUE TO DIESEL SPILLAGE
Diesel spills can occur because of crashes but tend to be more common in the vicinity of gas stations and garages for buses and lorries. It can be caused by overfilling of tanks or that the tank cap in trucks and buses are poorly fitted or damaged. This might lead to leakage in the first curve, intersection, or roundabout after the gas station. This is a common cause of motorcycle crashes since the spill is often not visible.

A study commissioned by the Sveriges MotoCyklister (SMC) in 2017 showed that diesel spills on asphalt lead to a reduction of friction from 0.8 to 0.3 (54). Data from motorcycle crashes in the UK 2005-2008 shows that diesel and/or oil spillage was a factor in 25-30% or 806 accidents where a motorcyclist was injured. The estimated cost for these injuries was £50 million. (127).

Possible solution
Diesel spillage not only led to loss of friction and crashes, it also damages road surfaces. Therefore, preventing spills is critical. Highways England has developed guidance for commercial vehicle drivers and operators to prevent diesel spills (61). This guidance includes educating the heavy goods vehicle on the environmental impacts and recommends daily inspection of vehicles and the carrying of spill kits. Other measures include fitting fuel caps with anti–syphon devices and self-sealing tanks and installing guard rails and other protections around the fuel tank to prevent damage.
6.8 MEASURING SKID RESISTANCE
Measuring skid resistance is an effective way to monitor the friction of the road surface and upgrade those roads if necessary. Maintaining good surface friction is critical for motorcyclist safety.

There are several instruments and methods which are commonly used, such as the Sideway-force Coefficient Routine Investigation Machine (known as SCRIM) and the British Pendulum Method in the UK. The present methods typically measure friction for specific parts of the road lane for twin track vehicles (usually the outer wheel track) in wet conditions. Motorcyclists, which are single track vehicles, typically use different parts of a lane to cars, especially when it is necessary to avoid rutting caused by cars and trucks. Furthermore, current approaches often exclude sealing and patch repairs (62). This means that skid resistance measurements may not be capturing the friction measurements needed for understanding the risk to motorcyclists. Furthermore, current approaches exclude sealing as well as patch repairs with bitumen (62). This means that skid resistance measurements may not be capturing the friction measurements needed for understanding the risk to motorcyclists.

Possible solutions
Consideration should be given to the measurement of skid resistance across the full width of carriageway, especially on the approach and exit of bends. The reporting of Skid resistance should be in discreet short lengths rather than average over 20-meter lengths. This will allow for more accurate consideration and auctioning of works where lengths of skid resistance are poor. It is possible to measure every meter and to use a friction average of for example two meters instead. New methods to measure the skid resistance of the road surface will have a potential to improve friction and reduce crashes. Partially this could be solved by making use of camera- and lidar techniques (3D scan) as are already used in small scale in several countries.

Another example is the sensor-equipped Austrian MoProVe motorcycle, which is used for risk analyses and safety inspections, based on collected vehicle dynamics data (53). The MoProve ia a rolling road infrastructure laboratory which makes it possible to create risk maps and risk models, based on processed data.

Current laser profilers can measure with resolutions smaller than 0.1 mm. These technologies typically collect measurements for both wheel paths and report an average at the 100 m level. Reporting individual wheel paths with an indicator that short poor performing sections would be a quick adaptation to cater for motorcycles. This needs to be further investigated (63).

6.9 ROAD ALERTS, WARNINGS, AND INSPECTIONS
The need for inspections of roads is essential. Who or which part that oversees the inspections varies between countries as well as the liability of the road conditions when crashes occur. Roads are monitored via cameras, road user reporting and the different road operators. Reports must be processed. There are several examples in Sweden where alarms were neglected, and poor friction led to loss of control among riders which ended with crashes involving lifelong injuries and fatalities (64).

Possible solutions
When there are fluids, debris, gravel, or other obstacles on the road it is important to have a warning system in place. This is usually a function owned by the responsible local, regional, or national road owner. Reports can be made via e-mail, apps, or phone. The motorcycle community can be invited to cooperate which has been a fact for many years in Norway and Germany (59).

The alert function must be aware of the potential risks that different obstacles on the road might have for motorcycles. The alert function must also take the alerts seriously, make sure that measures are taken to clean the roads to prevent crashes and follow up on the actions taken.

EXECUTIVE SUMMARY
CHAPTER 1. INTRODUCTION
CHAPTER 2. MOTORCYCLE SAFETY IN FIGURES
CHAPTER 3. GLOBAL, REGIONAL AND NATIONAL GUIDELINES ON MOTORCYCLE SAFETY
CHAPTER 4. WHERE DO MOST INCIDENTS AND CRASHES OCCUR?
CHAPTER 5. ADDRESSING MOTORCYCLE SAFETY
CHAPTER 6. ROAD SURFACE FRICTION
CHAPTER 7. THE NEED FOR ROADSIDE SAFETY ZONES
CHAPTER 8. BARRIERS AND MOTORCYCLISTS
CHAPTER 9. GUIDANCE BEFORE CURVES
CHAPTER 10. RECOMMENDATIONS
APPENDIXES
REFERENCES
TERMINOLOGY AND ACRONYMS
WORKING GROUP INFORMATION

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