This is a major public health concern and greater efforts are needed to educate the public and enforce these laws, argue the authors.
The study took place at three different sites in Hammersmith, West London. Private passenger vehicles were observed Monday to Friday for one hour in the morning (9-10 am), afternoon (1-2 pm), and early evening (4-5 pm).
The first observations were carried out in February 2004, within the "grace" period regarding use of hand held mobile phones, during which police only cautioned offenders. Observations were repeated one month later, after police began to impose penalties for non-compliance with the new law.
A total of 38,182 normal cars and 2,944 four wheel drive vehicles were included in the analysis. Overall, almost one in six drivers (15.3%) was not wearing a seat belt and one in 40 (2.5%) was using a hand held mobile phone while he or she passed the observer.
Drivers of four wheel drive vehicles were almost four times more likely than drivers of other cars to be seen using hand held mobile phones. They were also more likely not to comply with the law on seat belts.
Levels of non-compliance with both laws were slightly higher in the penalty phase of observation than during the grace period, and breaking one law was associated with an increased likelihood of breaking the other.
"Our data show a worryingly high level of non-compliance with laws on seat belts and hand held mobile phones by drivers in London, and almost no effect of the end of the grace period on the use of a mobile phone while driving," write the authors. "Our observation that almost one in six drivers was not wearing a seatbelt is a major public health concern," they add.
The findings also support the theory of risk compensation, which predicts that drivers of four wheel drive vehicles feel safer and therefore take more risks when driving.
Although four wheel drive vehicles are safer in a crash, their owners may be placing themselves and other road users at increased risk of injury, warn the authors.