Traffic congestion occurs when traffic demand exceeds road capacity, or when an incident such as a traffic crash, a vehicle breakdown occurs or temporary roadworks take place, all of which temporarily reduce capacity and restrict traffic flow. Congestion crashes mainly occur at the tail end of a traffic jam. There, the speed of the traffic flow decreases sharply, which coincides with frequent and hard braking, and with a high risk of rear-end crashes.

If the average speed on a road increases, crash risk also increases, as does the risk of a serious outcome. This is true in general terms, but more so when motorised vehicles crash with unprotected road users, such as pedestrians, cyclists and (light) moped riders. Furthermore, speed differences between vehicles at any one time or place are related to a higher crash risk. Drivers that maintain a speed that is higher than the average speed on that road run a higher crash risk; drivers that maintain a speed that is lower than average do not.

In 2020, over a quarter of the total number of bicycle kilometres were cycled on pedelecs; particularly the over-65s opt for pedelecs. This is also borne out by the crash figures: in 2019 and 2020, almost one in three of the cyclist fatalities was a pedelec rider.

Adolescence is the period between childhood and adulthood. Compared with older adolescents (18-24 years old), young adolescents (10-17 years old) exhibit more risky behaviour.  Because of their physical and mental development, young adolescents are attracted  to risky challenges, they are more susceptible to peer pressure, and they have less self-control and overview than older adolescents. This is also reflected in their road use. In comparison with other age groups, young adolescents, boys in particular, die relatively more often of unnatural causes; this largely comprises road crashes.

In this factsheet wrong-way driving is defined as ‘a car driving in the wrong direction on a road with separated driving directions and consequently driving into oncoming traffic '. This relates mainly to motorways. Wrong-way driving crashes are rare. The outcome, however, is often severe. Most wrong-way driving crashes occur when a driver enters a motorway exit ramp or when a driver reverses direction on a motorway. Orientation problems (especially among the elderly) or recklessness (especially among young drivers) are the most common causes.

Children are a vulnerable group among road users. They are, after all, still building up skills which will eventually allow them to become safe and independent road users. The role of parents in teaching their children how to behave safely in traffic is very important.  In this fact sheet, children are taken to belong to the age category 0 to 14, unless specified otherwise.

Risky road user behaviour is behaviour that adversely affects road safety, such as driving under the influence of alcohol, drugs or medicines, speeding, inappropriate speed, distracted or fatigued driving, red light negation, and failure to use or misuse means of protection (motorcycle or moped helmet, seatbelt). Younger road users more often display risky behaviour than older road users, and men more often than women.

The mobile phone is symbolic of ‘distraction in traffic’. But apart from mobile phone calls, texting, or listening to music, many drivers, cyclists and pedestrians are occupied with all sorts of other activities that may distract them. Examples are: operating the navigation system, eating, drinking, talking to passengers or daydreaming.

Driving under the influence of drugs or impairing medicines reduces fitness to drive[i] and increases crash risk. Drugs have a numbing, stimulating or mind-altering effect on the brain, or a combination of these effects, which impair traffic task performance. For drug use in traffic, we (unfortunately) have to rely on research dating back to 2011.

The weather has an influence on road safety. Weather conditions partly determine the road conditions and the driver's behaviour. Most studies into the relation between weather and road safety are about the situation during rainfall. However, many other weather conditions are serious influences: fog, snow and black ice, low sun, hard wind, and high temperatures.