Research has mainly focussed on the potential benefit of daytime running lights. Although it is clear that the perception of a PTW is much improved by the use of a PTW headlight during the daytime this is only a partial solution; many accidents still occur where the driver had not seen the motorcycle in spite of the headlight being on.
Research studies show that the use of daytime running lights by cars has no adverse effect on the benefits of headlight use by PTWs in most situations.
Daytime running lights for PTW's
Several experiments have been done on potential solutions to the problems of perception of PTW’s by other road users. The effect of headlights during daytime and of clothing have been studied by Hole et.al. using slide pictures and by Langham  using video presentations in a similar study. At short viewing distance it did not matter whether the motorcycle headlight was on or against which sort of background the motorcycle was shown. At longer viewing distance the motorcycle was noticed better with headlight on against a complex background. The effect of bright clothing also depended on the type of background. In another experiment Hole & Tyrrell  found that the more pictures were shown in which other motorcycles had headlights on, the less likely a motorcycle without headlight was to be noticed.
These experiments will never fully reproduce the natural behaviour of car drivers in real traffic i.e. their strategy to scan the road for other road users who may be relevant to them. But the assumption is made that if one condition (e.g. headlight on) is found to be better than another (e.g. dark clothing) this will be the same in real traffic.
The effects of headlights and clothing in practise have been studied in a case control study in New Zealand  with 463 accident cases from 1993-1995 and 1233 controls. The relative accident rate was corrected for other factors such as age and experience of the rider and found to be 27% lower for motorcycles with headlight on during daytime and 37% lower for riders with reflective or fluorescent clothing.
Bijleveld  used accident statistics from Austria and calculated a saving of 35% of collisions between car and motorcycle during daylight after the introduction of compulsory use of headlights by motorcyclists (compared to a situation with 0% use). Elvik and Vaa  included 12 primarily studies from the United States in a meta-analysis on the effects of a mandatory use of running lights. This meta-analysis showed a reduction of around 7% (+/- 3%) in the number of multiparty accidents in daylight.
Although it is clear that the perception of a PTW is much improved by the use of a headlight during daytime it is only a partial solution. Noordzij & Vis  still found many accidents in which the car driver had not seen the motorcycle in time in spite of its headlight being on.
Daytime running lights for cars
The benefits of motorcycle headlights may be less if cars also have their headlights on. There are some experimental studies on this subject. Brendicke et. al. showed pictures of traffic situations and observers had to report which vehicles they had seen. The pictures contained motorcycles with headlights on and cars with or without headlights on urban as well as rural intersections and road sections. A closer examination of their results shows that only on rural intersections were motorcycles less often noticed than cars and even less so in combination with cars with headlights.
In an experiment by Brouwer et. al.  observers had to report if and which other road user was present. The slides always showed a car with or without headlight. The experiment also varied the distance between car and other road user, the background, the proportion of slides with a car with headlight on and the proportion of slides with another road user. It took more time to detect a pedestrian than a pedal cyclist or motorcyclist and the detection of a motorcycle with headlight was even faster than without headlight. However, the headlight of the car had no negative effect in all situations studied.
In a field experiment  observers in a moving car had to report which road user they had seen at an intersection. There was always a car alone or together with a pedal cyclist or a motorcycle with headlight on. Observers always noticed the pedal cyclist and motorcycle except when the car had very bright headlights on.
The conclusion from these experiments is that the use of headlights during daytime by cars has no adverse effect on the benefits of headlight use by PTW’s in most situations, except when the car headlights cause glare. This conclusion is confirmed by Koornstra et. al.  with accident statistics from Norway and Denmark where compulsory use of headlights was introduced for cars some years later then for motorcycles. An increase in collisions between car and motorcycles during daylight was insignificant.
All studies were concerned with motorcycles rather than mopeds, but the perceptual problems are likely to be the same and the use of headlights during daytime by mopeds and the wearing of fluorescent/reflective clothing is a partial solution to them as well.
Another contribution to solve the problems of perception of PTW’s may be found in more attention to these problems in the training of car drivers.