30 km/h blanket speed limit – does it work?

Effects of blanket speed reductions in Graz to 30 kmh/50 kmh Summary The introduction of blanket speed reductions of 30 […]

Effects of blanket speed reductions in Graz to 30 kmh/50 kmh

Summary

The introduction of blanket speed reductions of 30 kmh on side streets and 50 kmh on major roads hat an overall positive effect.

–          Traffic safety increased

–          Average speeds declined slightly, but traffic movement was more homogenous

–          Behavior of traffic participants improved, leading to a subjective improvement in safety for vulnerable road users

–          Noise pollution decreased somewhat, whilst environmental pollution was not much affected

–          The acceptance of lower speed by motorists increased substantially

Effects on cyclists:

In the period from 1992 to 1999 cycling trips per day increased from approx 110’000 to 125’000, but accidents with cyclists involved declined from 490 to 380.

 

Details

Graz (Austria), a city of over 300’000 inhabitants, whose most famous export is Arnold Schwarzenegger, introduced a speed limit of 30 km/h for suburban streets in 1992, with most major roads having speed limits of 50. In 1995 G.Sammer and colleagues at the Technical University of Graz produced a detailed and documented before/after study

  1. Introduction

It was found that speed reduction reductions in small steps would be expensive and would lead to a “forest of traffic signs”. Graz decided to trial blanket speed reductions, which would be signaled at the edge of the town. The signs indicated a blanket speed of 30 kmh except on priority roads. And even priority roads were speed reduced, for instance when they passed schools. The trial started in 1992, was evaluated in 1994 and because of the positive results, the blanket speed reductions were made permanent.

  1. Framework

The total road network in Graz is 1004 km, of which 229 km are priority roads and  775 km of secondary roads. Secondary roads account for only 10% of total vehicular traffic. Secondary roads have one or two lanes and where they cross sightlines are small. The accident risk on secondary roads (compared to distance travelled) is three times higher than on priority roads.

When the discussion around speed reduction started in 1992, the approval for lower speeds was around 44%, but by 1995 this had nearly doubled to 82%. The lower speeds were monitored by the police with additional speed guns and electronic signs indicating driven speeds.

  1. Observation of traffic behavior

Driver behavior is influenced by driver attitudes, road appearance, vehicle technology, traffic routes and volumes, which in turn influence driven speeds as well as crash frequency and severity.

To observe the traffic behavior both qualitative and quantitative measures were used to evaluate route choices and traffic adaption, using both direct observations and filmed material. At the same time detailed speed measurements were taken of vehicles, bicycles and pedestrians approaching, transversing or departing the crossings under observation.

The data was used to build behavior models and examine perceptions of objective and subjective road safety of traffic participants. Objective safety is the ability to react, based on vehicle characteristics, road design and conditions, traffic volume and speed and the behavior of other traffic participants. Subjective safety is the preparedness to react, based on how other traffic participants act, own interpretation of events, subjective safety evaluation, the acceptance of lower speed, and driver concentration. Together the ability to react and the preparedness to react result in the probability of appropriate reaction, which in turn influences the crash probability.

Appropriate traffic behavior assumes that objective safety is as high or higher than subjective safety. Acceptance of preventative safety measures makes them more effective.

The report analyses 1357 events, of which 565 preceded the introduction of the blanket speed reductions. It was found that the measures resulted in lower speeds and more homogenous traffic flows and a safer overall traffic environment. Motorists and other traffic users became more likely to adjust their behavior to each other. Despite cyclist generally using the roads in a more predictable manner, overtaking of cyclists halved:  motorists would avoid overtaking of cyclists, instead they would follow without overtaking.

  1. Choice of routes and mode of transport

Based on a small sample analysis of 3000 km travelled, it was concluded that the shift of route choices from 30 kmh zones to 50 kmh zones was negligible – only 1.5% of trips moved to the higher speed zones. This was insufficient to change traffic flows in either speed zones.

  1. Pollution.

Over the whole area of Graz the reduction in pollution was insignificant, mainly because only a small proportion of the total kilometers travelled take place in the 30 kmh areas. However in these lower speed areas the reduction of nitrogen oxides produced during combustion was significant.

  1. Noise pollution

Depending on location, noise levels declined by 0.4 dB (hardly noticeable) to a significant 1.9 dB. This is attributed to the lower speeds as well as the more even car speeds.

Summary:

The introduction of blanket speed reductions of 30 kmh on side streets and 50 kmh on major roads hat an overall positive effect.

          Traffic safety increased

          Average speeds declined slightly, but traffic movement was more homogenous

          Behavior of traffic participants improved, leading to a subjective improvement in safety for vulnerable road users

          Noise pollution decreased somewhat, whilst environmental pollution was not much affected

          The acceptance of lower speed by motorists increased substantially

Effects on cyclists:

In the period from 1992 to 1999 cycling trips per day increased from approx 110’000 to 125’000, but accidents with cyclists involved declined from 490 to 380.

 

 

References:

 

In 1995 G.Sammer and colleagues at the Technical University of Graz produced a detailed and documented before/after study. On 120 pages the paper examined the effects of the blanket speed reductions on traffic flow, driver behavior, route choices, fuel use and pollution, noise pollution and effects on cyclists and pedestrians. At some later date M. Hoenig of the City of Graz used the paper as the basis of a report on the effect of the speed reduction for cyclists.

This report focuses on the effects of blanket speed reductions on vulnerable road users in a suburban context, and is based on the work of both, but is neither a complete translation nor a complete summary.

Sammer, Pischinger,Schützenhöfer et.al.30/50 speed limit in Graz; results of the scientific accompanying investigation into the trial model, 1995

And to a lesser extent by Manfred Hoenig, Department of transportation, City Council Graz, A 8011 Graz, Austria manfred.hoenig@stadt.graz.at

 

 

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