Over hundreds of thousands of years, humans have evolved to move at 20mph for umting. Our focus is very narrow covering only 2 degrees of out view. We are aware of up to 180 degrees outside of out focus. This is so humans can both focus on the hunt and whether they are being hunted.
These adaptations are not ideal for modern fast travel so to understand what is happening around us, people have to move their eyes. This is done at a constant rate of 3 to 4 movements per second.
The issues come when the driver has to dwell on an object to understand it or if they find it interesting. At the same time the vehicles is driving along the road. Three eye movements per second, at the urban legal speed limit of 30mph, equates to one eye movement every 13.5 metres. If a driver looks awat for 2 seconds, they could not see anything else on the road for 80 metres. This is sufficient to fail to see a child or cyclist on the road.
Even if drivers are normally able to physically see something on the road there may be factors preventing them from noticing it. Usisally, to compensate for the considerable limitations of the eye, the brain works closely with the eyes, fill in the gaps, based on past experience and make assumptions so that the driver can usually correctly understand what is ahead. In addition, because there is so much to look at nd no time to focus on everything, experienced drovers become very good at looking out for the right things. Unfortunatly, people do not tend to see something that they are not actually looking for, even if it is large and colourful, or even a cyclist.