Evaluating the effect of enforcement on speed distributions using probe vehicle data

- Analysis of spatial and temporal impacts of cameras on speeds.
- Use GPS probe vehicle data for safety evaluation.
- Consider 22 installation sites and 12 comparison sites.
- Speed reductions of 2.92 km/h (short-term) and 6.48 km/h (long-term).
- A-symmetric double-exponential model depicts halo effect.

Collecting driving speed data is a crucial element in road safety research. Recent technological developments enable a wide range of alternative traffic monitoring possibilities. In particular, the availability of cellular and GPS devices creates the opportunity to utilize probe vehicle data. To date, most studies using probe vehicle data focused on operational purposes such as congestion identification, relying primarily on average speeds.Probe vehicle data can also be useful for safety studies. The data is often collected continuously (24/7, 365 days a year). It can therefore enable detailed analyses of the continuous changes in speed over time. In addition, the spatial coverage of the data enables continuous evaluation of the halo effect of speed enforcement installation, compared with a relatively small number of measurement sites (e.g. 3-5) in traditional methods.Probe vehicle data offer one more advantage: When performing a before-after study, the before measurements have to be performed before camera installation. However, installation plans may change, and therefore conducting the before measurements may be administratively challenging. Probe vehicle data, especially if collected continuously, regardless of research needs, enable researchers to choose the desired sites and times for analysis in retrospect, after the cameras have been installed.The purpose of this paper is to describe the results obtained from analysis of probe vehicle data on the effect of speed enforcement cameras that were installed at non-urban roads in Israel during 2011-2013. In addition to the contribution regarding the impact of enforcement cameras, the paper demonstrates the potential (as well as the challenges) of using probe vehicle data for safety studies. The analyses in this paper consider 22 installation sites and 12 comparison sites. The results show that at the installation sites (within 100 m), the 85th percentile speeds exhibited a short-term (two months) reduction of 2.92 km/h (SD = 2.37) and a long-term (one year) reduction of 6.48 km/h (SD = 3.5). An a-symmetric double exponential model is presented to depict the halo effect up to 1.5 km from the installation location.