A recent study evaluated speed display signs at pedestrian crossings in a low-speed (40 km/h) urban environment. Installing speed displays significantly reduced the mean speed, proportion of speeding vehicles, and approaching speed at pedestrian crossings. The speed displays were installed for 5 months and the decrease in speed persisted over time, suggesting that they may reduce speeds in the long term.
Reducing driving speeds has been recognised as the one of the main measures for improving traffic safety for all types of areas and road users. Driving speeds can be reduced e.g. by installing roadside systems such as speed displays, which are interactive signs showing a vehicle’s approaching speed (figure 1). Their main purpose is to inform the driver of their actual speed and thus improve adherence to the set limit and reduce speeding. Speed display signs have been assessed to reduce driving speed e.g. in speed transition zones, horizontal curves, work zones, school zones and high-speed urban areas. Lately, they have also become common in low-speed urban areas; however, fewer studies are available on evaluating their effect in such areas. Hence, this study examined the short- and long-term effects of speed displays on driving speeds on urban streets with low speed limits (40 km/h).
Overview of field study
The design was a before-after study with control sites. Speed effects were evaluated in two street environments – busy and quiet collector streets connecting local streets to arterial roads – each with two study sites in southern Finland. The study also included two control sites, one for each street environment. All sites were located on straight two-way street sections with one lane per direction, no horizontal variation, and a posted speed limit of 40 km/h. The speed displays were fixed to light poles 45–65 m before a pedestrian crossing on a straight road section. Driving speeds (spot speeds) were measured roughly 100 m before and at the pedestrian crossing. Driving speeds were compared 1 week before installation, after installation (1 week; 1 month; 3 months; 5 months) and 1 week after removal.
Significant reduction in spot speed
The main speed analysis compared the spot speed of all vehicles at a pedestrian crossing. While the speed display was installed, the mean speed of all vehicles dropped by 0.5–2.9 km/h. The effect on the proportion of vehicles speeding was -17…+1 percentage points, and on the proportion of those speeding by more than 10 km/h -14…+6 percentage points.
Lesser effect on drivers selecting their own driving speed
The second speed analysis compared the approaching speed of individual vehicles travelling in free flow traffic (with a headway of 6 seconds or more). While the speed display was installed, the approaching mean speed dropped by 0.2–0.8 km/h at two sites, increased by 0.4–1.2 km/h at one site and mainly remained the same at one site. Hence, speed displays have a lesser effect on drivers selecting their own driving speed than on the overall traffic flow. The results also show that the speed effects varied somewhat by site and phase, implying that although the effects are generally positive, there is room for further research.
Speed effects persisted over time
While the speed displays were installed, driving speeds remained lower over time, suggesting that speed displays can have a long-term impact on driving speeds. However, the mean speed exceeded the speed limit during all measurement phases.
The study was carried out by VTT as part of the consortium programme Traffic Safety 2025.
Text: Fanny Malin, VTT
Fanny Malin & Juha Luoma (2020). Effects of speed display signs on driving speed at pedestrian crossings on collector streets. Transportation Research Part F 74, 433-438.
VTT Technical Research Centre of Finland Ltd, Finland