Ask any average group of drivers what they think of speed humps and the answers are usually taken from an eerily similar set of unflattering adjectives – irritating; infuriating; unnecessary. Although their effectiveness in reducing vehicle speed has been proven over time, motorists are consistently irritated by the stop-start driving that speed humps require, as well as incremental damage sustained to vehicle undercarriages.

More seriously, physical traffic calming measures such as speed humps and chicanes have been criticised for impeding emergency services vehicles and increasing their response time. Many residents also complain about increased traffic noise.

Given the level of popular objection to physical speed reduction methods, many towns and cities are looking for an alternative that will reduce speeds in residential areas while keeping disruption to a minimum. One such town in the US – Bellevue – may have found a solution.

Radar speed signs

Radar speed signs, also known as driver feedback signs or vehicle-activated signs, combine a digital display (usually a series of LEDs) with speed tracking technology to alert drivers if they are driving at dangerous or illegal speeds. Evidence of the effectiveness of these signs is building and they are now becoming a common feature on roads around the world.

“Many towns and cities
are looking
for possible alternatives that will reduce traffic speeds while keeping disruption to
a minimum.”

In the summer of 2009, the Transportation Department of Bellevue, Washington, issued a report on its experience of radar speed signs as a traffic calming measure. The rapidly growing urban centre, which lies on the opposite bank of Lake Washington from Seattle, has been steadily increasing its use of radar speed signs since 2000.

Officials began installing radar speed signs to help the police and address the concerns of community groups on roads where physical measures were unfeasible. After mounting evidence of the signs’ effectiveness, and increasing demand from residents and the Bellevue Fire Department, city authorities have begun to place radar signs even on roads that would qualify for physical traffic calming measures.

As of Bellevue’s report last year, the city had set up 31 stationary radar signs from a range of manufacturers. After a year of evaluation, the report found that on the roads where the radar signs were placed, 85th percentile speeds (the speed at or below which 85% of vehicles travel on a given section of road) were generally reduced by at least 5%.

It was feared that the speeds would increase once drivers became accustomed to the signs, but the report found the opposite: the signs have become more effective over time. In locations where radar signs had been installed for six or more years, the majority of 85th percentile speeds dropped by 10% or more.

This evaluation period has clearly left Bellevue traffic authorities with an overwhelmingly positive impression of radar signs, as the 2009 report notes in its conclusion: "Although educational in nature, these signs are effective at bringing motorists’ attention to the speeds they are driving. Evaluations of the signs have shown that a significant number of motorists are choosing to correct their speed when necessary."

The case for radar speed signs

“Radar signs can also be mounted on vehicles
for rapid deployment.”

Bellevue’s experience is echoed by growing body of evidence on the effectiveness of radar signs in reducing traffic speeds, with reports by the Transportation Research Board, the UK Department for Transport and several studies by US states adding weight to Bellevue’s findings. Beyond their efficacy, these devices have a number of distinct advantages over speed humps and the like.

Most importantly, radar speed signs address the major problem that many critics have with traditional speed humps; without any physical obstacle on the road, ambulances and other emergency response vehicles can drive as fast as required to reach their destination and potentially save lives.

Another key benefit of radar speed signs is the flexibility they offer to transportation authorities. Compared with planning and constructing a series of humps, radar speed signs can be put in place quickly and easily. And unlike traditional calming measures, if a speed sign is proving unpopular or ineffective it can be easily relocated to an area of greater need with little fuss.

Radar signs can also be mounted on vehicles for rapid deployment. In Bellevue’s case, this technology has been a great way to get local communities involved in traffic calming programmes. Bellevue residents can borrow a portable radar unit from the city and place it on an appropriate road for up to two weeks to deter speeding motorists.

An all-radar future for traffic calming?

Despite the clamour of voices singing the virtues of radar signs and their effectiveness at reducing traffic speeds, the fact remains that there are significant drawbacks.

For all their advantages, radar speed signs are not always a viable replacement for existing strategies. Although the Bellevue report is a glowing endorsement for the technology, the document also reveals its limitations.

was a recurring problem at several of the sign sites.”

The report lays out a list of criteria for proper radar speed sign placement. This list makes it clear that it’s not possible to simply position a radar sign on any road and expect to achieve positive results. The signs must be placed with at least 300ft of clear sight distance, streets with metal objects (parked cars, for instance) can interfere with the signs’ radar signals and light intrusion from LED displays through the windows of nearby homes at night can also be a problem.

Even when signs are optimally placed there are unique drawbacks that don’t affect traditional speed humps. The Bellevue report notes that vandalism was a recurring problem at several of the radar sign sites: "The most common types of vandalism that occurs are objects (paintballs, watermelons, pumpkins, eggs, milkshakes, bricks, rocks, etc) being thrown at the display. At the extreme end, vandalism by firearms has also occurred."

The case for radar speed signs is also undermined somewhat by the fact that a physical solution to the question of obstructing emergency services has been found. Speed cushions have seen widespread adoption, as their separated road bumps slow down cars while allowing wide-axle emergency vehicles such as ambulances and fire engines to pass through without slowing down. Given that after the initial investment, speed cushions require significantly less ongoing maintenance (and no power consumption) than radar signs, this has been a more appealing option for many local authorities.

So while radar speed signs have been universally praised for their contribution to traffic calming efforts, certain limitations prevent them from becoming a true alternative to existing measures. It seems likely that towns and cities like Bellevue will continue to adopt this technology as a central part of their overall traffic calming strategy rather than as a replacement for it.