Factors related to the vehicle
The effect of engine performance on safety is not fully understood; the relation between engine power and accident rate is not a simple one. And engine cubic capacity is not a very good indication of motorcycle performance and its associated accident rate. In practice the relation is further complicated since different groups of PTW’s are used by different groups of riders with different accident rates.
Accident rates differ between different types of PTW.
Mopeds with their small engine and restricted top speed have accidents with less severe injuries than motorcycles, which results in lower fatality rates. Including less severe injuries results in rates that are not much different from or even higher than for motorcycles.
There is little known about the accident rate of 125cc motorcycles.
Sports motorcycles have been found to have higher accident rates than other types of motorcycle. Power to weight ratio is probably a better indication of performance and more strongly related to the accident rate of sports motorcycles than cubic capacity. A higher accident rate for these motorcycles, even if corrected for age and experience of the rider, does not automatically mean that the type of motorcycle is more difficult to control and therefore less safe. It is quite possible that they are used by riders with a different style of riding. There is no knowledge on the accident rate of scooters either as moped or as motorcycle. This is unfortunate because their number on European roads is increasing.
Breaking a PTW is difficult and loss of control in emergency situations often occurs.
At first sight the engine power seems to be the most important of all PTW properties in relation to safety. There are several studies on this subject with mixed results. The idea is that a heavy and powerful PTW is difficult to control even at low speeds and may invite riders to test the potential acceleration and/or top speed, which brings them in situations which are difficult to control. This means that it is not necessarily the character of the PTW itself, but the experience and motivation of the rider which determine the safety of the rider-PTW combination. In addition, heavy or powerful PTW’s may be used more in conditions which differ from smaller PTW’s and these conditions themselves may be related to the safety of PTW use.
A study with accident data from 1984-1986 in Great Britain covers the whole range of engine (cubic) capacity. The kilometre data came from a travel survey of 372 riders . Accident rates per million kilometres were found to be related to the age of the rider and traffic conditions as well as to engine capacity. Riders of 16-18 years had fatality rates which were about four times higher than for riders of 30 years and older, after correction for conditions (built-up or non built-up) and engine size. The age difference was even stronger for less severe injuries. The fatality rate for non built-up roads was about one third higher but the rate for less severe injury about half that for built-up roads. This is an indication of more accidents on built up roads (corrected for kilometres) but with much less severe injuries. After correction for age of the rider and traffic conditions, the fatality rate for the 50cc PTW was lowest and highest for PTW’s with engines of more than 250cc, with the fatality rate for 125cc in between. Including injuries resulting in hospital admission produced a rate which is a little higher for 125cc machines with little or no difference between the other engine capacities, i.e. both 50cc and over 125cc. It must be remembered that in this study most riders of 125cc PTW’s had a provisional license. Problems with the interpretation of these results are the small sample of the travel survey and the absence of information on the experience of the PTW riders. The study is rather dated and to day’s motorcycles in general have bigger and more powerful engines.
The “Handbook of road safety measures”  refers to a Norwegian study by Ingebrigtsen  which corrects for age, experience and other factors, including a measure to take risk. The study found no substantial increase in accident rate with increasing cubic capacity.
A more recent New Zealand study used 463 accident cases from 1993-1995 and 1233 controls in a case control design . The relative accident rate was corrected for age and experience of the rider and other factors and was found to be somewhat lower for motorcycles under 250cc, with no clear relation between cubic capacity and relative accident rate for motorcycles over 250cc. The authors conclude that if cubic capacity is used as basis to restrict motorcycles of novice riders the limit should be substantially lower than 250cc and power to weight ratio or motorcycle type may be a better basis.
Power to weight ratio
The cubic capacity of the engine may not be the best indication of the potential acceleration and top speed. An Australian study used power to (laden) weight ratio for the motorcycle and focussed on novice riders . There were 2247 novice riders involved in casualty accidents in 1987-1990 (learners: 1356 and first year license holders: 891). The age limit to obtain a license was 18 years. The sample of motorcycles was compared with registered motorcycles. The accident rate (corrected for estimated kilometres) shows a strong relation with power to weight ratio, with four times higher rates for the most powerful motorcycles compared to the least powerful. The study did not correct for age of the rider, but many of the novices must have been young. It has to be remembered that learner riders were restricted to motorcycles with engines up to 260cc.
Type of PTW
There is a recent study with a case control design with samples from mostly urban areas in five European countries: France, Germany, the Netherlands, Spain and Italy . The accident sample for 1999-2000 contained 398 mopeds and 523 motorcycles, which were compared with 923 control cases. The report states that there is no difference between the accident and control cases with regard to age of the PTW rider, moped versus motorcycle and scooter versus other PTW. Young moped riders and scooters with mostly young riders are compared with mostly older motorcycle riders. Unfortunately, this does not allow conclusions to be drawn for these factors separately.
Mopeds with their small engine and restricted top speed can be expected to have a lower accident rate than motorcycles. But this is not evident from the actual figures. They have accidents with less severe injuries than motorcycles, which results in lower fatality rates. Including less severe injuries results in rates that are not much different from or even higher than for motorcycles. There are some remarkable differences between countries. A report comparing the traffic safety in the United kingdom, Sweden and the Netherlands discusses the difference in moped accident rates (Sunflower).This rate (over all ages) for the Netherlands is almost double that for the other two countries. In the United Kingdom the moped is much less popular than in the Netherlands but the average kilometres per moped per year in the United Kingdom is about double that in the Netherlands. Therefore the use of the moped must be quite different. In the United Kingdom the moped is mainly used for commuting, in the Netherlands more for social or recreational purposes and in Sweden the moped is mainly used in the summer. In Great Britain moped riders have to obtain a license after taking a basic training course and passing a test (car drivers only have to take the training course and motorcyclists are already qualified). There is also a difference in traffic conditions, which are much less dense in Sweden. All these differences may affect the overall moped accident rates. In addition, the majority of mopeds in Sweden are light mopeds with a lower maximum speed. The Netherlands have the same type of moped, but without helmet. According to Noordzij  the accident rate per million kilometres (and corrected for age) for the light moped without helmet is about the same as for the faster moped.
There is little known about the accident rate of 125cc motorcycles. At least in Great Britain they were shown to have a slightly higher rate than other motorcycles. This does not necessarily mean that this also holds for other countries. A German study presents fatality rates per 100000 motorcycles for 1994-1999 . For all age classes these rates are lower for 125cc vehicles only than for all motorcycles. For 125 motorcycles the age 16-19 has a fatality rate five to ten times higher than riders 25 years and older. There is no information on kilometres and the pattern of accident rates corrected for kilometres may be quite different. It is unfortunate that these rates are not known especially for the age group 16-18 and for the older drivers who are allowed to ride these motorcycles with only a car license since there are differences in legislation between countries for these groups.
The type of motorcycle has been studied in surveys in Germany . The samples were attendants of motorcycle shows. The questionnaires included questions on riding style and type of motorcycle. Only for sports motorcycles and only for riders with a sporting style the number of self-reported single vehicle accidents was higher for very powerful engines. There was no such relation for collisions with other road users.
With a different type of study design Noordzij & Vis  calculated accident rates per million kilometres for three groups of riders and three types of motorcycle. 926 motorcycles involved in accidents resulting in hospital admission in the Netherlands in 1993 were combined with kilometre data from a national travel survey with 3000 motorcyclists. The accident rate per million kilometres for sports motorcycles was about double that for touring motorcycles regardless of age and experience of the rider. Custom motorcycles also had higher accident rates than touring motorcycles, but only for inexperienced riders. This last finding is surprising because this has not been found in other studies.
Other design elements
Other design elements of PTW’s are relevant for their safety such as frame, suspension, wheels, brakes, tyres. But with to day’s PTW’s their contribution to accidents is low. The MAIDS study found 5% accidents with vehicle failure as a contributing factor, mostly tyre or wheel problems.
Breaking a PTW is difficult for several reasons. With only two wheels in line the PTW may easily loose friction between tyres and road surface resulting in a fall. This is more likely during braking or cornering and even more likely during braking in a curve. The rider has to carefully apply braking force taking account of the quality of the road surface and the leaning angle of the PTW. Modern high performance motorcycles have high performance braking systems with which locking of the wheels is easy. In addition, the PTW rider has to divide the braking capacity between front and rear wheel. Most PTW’s have separate controls for the front and rear wheel brakes. As a result the total braking capacity will seldom be fully used or, when braking suddenly, the rider may loose control.
Loss of control in an emergency situation is often found in accident studies . In a special study on the role of braking in accidents, Sporner  used a sample of 502 injury accidents in 2001-2002 in Germany. 279 of the motorcyclists took action to avoid the accident, of which 54 lost control.