Ads, signs or billboards that clutter roadsides can be hazardous for older drivers, according to the latest study.

These distractions delay drivers' ability to detect a change around them, especially when a vehicle changes lanes - by an average of half a second. Older drivers took the longest to react.

The finding by Monash University Accident Research Centre (MUARC) has important implications for the design and regulation of road environments, and could lead to a new debate around the need for tighter advertising restrictions on roadsides.

'Driving on a typical major road is a complex activity, where drivers must process large amounts of visual information which continuously changes, and make decisions at speed,' Monash researcher Jessica Edquist said.

'The research is very clear: as drivers we can only look at and pay attention to one thing at a time. When we are looking at a sign or a billboard, we are not looking at the road, leading to a higher accident risk.'

Edquist conducted a series of tests with more than 100 drivers, almost half using MUARC's high-tech advanced driving simulator. She found drivers were distracted by billboards - they drove more slowly, took longer to change lanes in response to road signs and made more errors when changing lanes.

Older drivers in particular had difficulty detecting changes on the road and in following road sign instructions in busy environments. The finding is crucial as, due to an ageing population, there are more people aged over 65 and more are staying on the road despite age, said a Monash University release.

Edquist said road authorities should carefully regulate billboards, declaring billboard-free distances around areas of high driver workload such as intersections, merges and freeway exits.