4.2.1 General problems with the time driven approach
Various methods have been used for esti-mating the effect of ISA in various trials.
Some results are based on the time driven while others are based on the distance dri-ven. Results are diverse and it seems that results from trials where results were esti-mated on the basis of the time driven data
4 Methods
are markedly smaller than those based on the distance driven. However, that might be caused by a number of other reasons such as generally different traffic beha-viour in different countries, different sorts of ISA systems, and different attitudes to speeding among the drivers. How these differences might affect ISA results is un-known and it is therefore useful to com-pare results estimated by the time driven and the distance driven, respectively.
Research on subjects involving speed, there are basically two approaches: a time driven and a distance driven one used in most cases. When measuring a single re-action it might often be useful to adopt a time driven approach in dealing with the incident because most road users use the time to a certain subject to assess the most suitable reaction then (Triggs, Harris 1982). It is also the type of approach used in conflict study techniques (Williams 1981). However, if studying speeding in relation to ISA, the use of time above a certain speed as measure can result in a systematic bias. A hypothetical example of that follows below. measured as less serious. So, in theory, if a vehicle drives excessively fast, no time will be spent on the road and it will be a completely safe trip. The problem can be illustrated with a practical example:
A car is driving a distance of 1 kilometre on a 60 km road. The vehicle speed with-out ISA is 75 km/h whereas it is 66 km/h with ISA activated. If using the equation above it will look like this:
Without ISA vs. With ISA:
When the speed is reduced by 9 km/h due to ISA, the time of speeding will increase by 7 seconds. Hence, the reduction of the larger speed limit violation will be mea-sured as an extended period of speeding (from 48 to 55 seconds) if only looking at the time driven above the speed limit.
4.2.2 Speed distribution
To avoid any unknown bias caused by lo-cal conditions, FCD from two ISA trials are studied regarding results based on the time driven and the distance driven ap-proaches. By distributing FCD on speeds any overestimation of e.g. low speeds can be recognized. Also differences in the distribution become distinct. FCD from the TAC Safecar and the ISA C trials re-garding 80 km roads with ISA activated are used for this study. Data from the two trials were selected, partly because they were available, but also because the tri-als are fairly new, included the same type of driving (commercial vehicles), and
in-cluded almost the same amount of trans-portation21. See Figure 4.1.
In both data sets it appears that low speeds are considerably overestimated when measured by the time driven approach. It is particularly significant in the case of the ISA C trial. The reason for this difference is probably another use of the cars than in the TAC Safecar trial (workmen vs. sales-men). However, regardless of the diffe-rent groups of users the low speeds are extensive and hence any effect of ISA will be underestimated. This underestimation is especially caused by very low speeds originating from idling, red light stops, and congestions, which by no means can be affected by ISA.
21 In the TAC Safecar trial each driver drove 16,500 km. 3,000 km of this distance was driven under baseline conditions without ISA activated while other 3,000 km were driven with ISA activated. With 15 drivers un-der ISA conditions the total number was 90,000 km deducted any erroneous data. The remaining 10,500 km driving per driver was used for other tests than ISA only. (Regan et al. 2006a). ISA C FCD from the baseline period are here compared with FCD from the following 1.5 months with ISA, hence referred to as the ISA pe-riod. During these two periods the participating vehicles drove 88,000 km in total (Agerholm et al. 2008b).
22 Violation of the speed limit by 2 and 5 km/h was the threshold for activation of warning ISA in the TAC Safecar trial and in the ISA C trial, respectively. Hence these values are used for the estimations.
It could be argued that removing the lo-west speeds would solve the problem. It will undoubtedly solve the bias caused by idling etc. However, the time driven ap-proach will still cause a bias because ef-fects from the low speed will be overes-timated. By removing say all speeds <5 km/h the bias will be minimised but not eliminated.
4.2.3 PDA and PTA
Also a comparison of PDA and PTA results for various road types can indicate the dif-ferences between the two approaches. FCD from the TAC Safecar and the ISA C trials distributed on speed limits are used to com-pare them22 (see Tables 4.1 and 4.2). The re-sults in each of the two tables are based on exactly the same method, though estimated on the basis of the distance driven and the time driven approaches, respectively.
Figure 4.1. The proportion of time and distance driven on 80 km roads in the two ISA trials. Left: dis-tributed on speeds for the TAC Safecar trial. Right: disdis-tributed on speeds for the ISA C trial. Note that the time driven approach results in major fluctuation regarding 0-2 km/h (18 and 69% of the total time driven, respectively).
4 Methods
When comparing the results based on the distance driven with the ones based on the time driven, it is evident that the former show lower p-values than the latter. How-ever, it only results in a single case of changed level of significance: in the TAC Safecar results on 100 km roads a dis-tance driven approach shows a tendency towards statistical significance, while no significance is found in the time driven ap-proach. The decreased difference, which is a consequence of a higher speed limit, may be due to the fact that the amount of idling and driving in congestions will tend to be less when the speed limit is higher. Hence, in the case of roads with high speed limits the two approaches would be most easily compared with each other, but it is here that a different level of significance is ob-served. Consequently, the values in Table 4.1, and partly in 4.2, indicate that the dis-tance driven approach gives more clear re-sults than the time driven approach.
Table 4.1. The PDA and PTA+2 km/h in the TAC Safecar trial.
Table 4.2. The PDA and PTA+5 km/h in the ISA C trial.
4.2.4 Selection of the distance driven approach
On the basis of the studies above it was decided to use the distance driven data for further analyses of change in driving be-haviour due to ISA. Three reasons seem to speak in favour of the distance driven proach: 1: in adopting a distance driven ap-proach any bias due to e.g. low speeds and idling is avoided. 2: in deselecting the dis-tance driven approach some of the results which should have been treated as statisti-cally significant would not be. 3: also, the distance driven is the normal measure of exposure to on-the-road risk (Elvik et al.
2009, Brems, Munch 2008).