Pedestrian crossings do not allow older people enough time to cross the road, a report warns.
The study found that for those over the age of 65, 76% of men and 85% of women have a walking speed slower than that needed to use a pedestrian crossing.
This speed is set by the Department for Transport at 4ft (1.2m) per second - an international standard.
The work, published in the journal Age and Ageing, calls for current pedestrian times to be reviewed.
Local transport minister Norman Baker said: "In my experience, the vast majority of people, young or old, get across the road as quickly as they can.
"The department recommends that where a crossing may be used by a large number of older people or those with mobility issues, for example outside residential care homes, this should be taken into account in the timings set by local authorities."
Dr Laura Asher, report leader and public health expert at University College London, said: "Walking is an important activity for older people as it provides regular exercise and direct health benefits.
"Being unable to cross a road may deter them from walking, reducing their access to social contacts and interaction, local health services and shops that are all important in day-to-day life.
"Having insufficient time at a road crossing may not increase the risk of pedestrian fatalities but it will certainly deter this group from even trying to cross the road."
She added: "The groups of people identified in this study as the most vulnerable were those living in deprived areas - those least likely to have access to other, more expensive, forms of transport."
The study used walking speed tests from around 3,000 older adults performed by the Health Survey (HSE) for England in 2005.
The participant's normal walking speed was assessed by timing how long it took them to walk 8ft (2.5m) at their normal pace.
The average walking speed for older men was 3 ft (0.9m) per second and 2.6ft per second for older women.
Dr Asher said: "By testing people in the general population rather than those already using a pedestrian crossing, we have included people who may have difficulty using a pedestrian crossing and are therefore unwilling to use them."
The lights at pedestrian crossing have a number of stages.
On an average road, the green man will stay lit for around 4 seconds. It then has a flashing or blackout stage for around 6 seconds. For roads more than 20ft (6m) wide, an extra second is added for each additional 4ft (1.2m).
After this there are an extra few seconds of an all-red light as a safety margin.
Peter Box, chair of the Local Government Association's economy and transport board, said: "The formula councils use for timings at pedestrian crossings actually gives people far longer to cross the road than this flawed research suggests.
"Timings have to strike a balance between traffic flow and pedestrian safety, but the emphasis is always on safety."
He said timings could be altered to take local circumstances into account, for example if there is a care home nearby.
To help pedestrians cross the road, Transport for London had been introducing "pedestrian countdown" technology to the capital.
This aims to help those crossing the road to be able to better judge whether they have enough time to do so safely - a digital display counts down between the end of the green man signal and the red man appearing, rather than just a flashing green man or blackout period.