Tema de la Propuesta:

One of the objectives of this thesis is to evaluate the impact of a potential speed limit increase, from 70 to 80 mph, on accidents. Apart from explaining the relationship of accidents with traffic and geometry related variables, the developed models can be em- ployed for impact estimation. Using the elasticity of accidents with respect to speed it is possible to estimate the expected changes in the number of accidents as a result of a motorway speed limit increase. For this purpose it is meaningful to employ the most detailed models for the accident-speed relationships which are the models that examine

accident occurrence by collision type and severity (i.e. SV KS-Sl and MV KS-Sl models). As it has been explained in Section 5.4 the models that will be considered for the impact estimation are the condition-based models only.

Speed limit increases typically lead to proportional average speed changes that are be- lieved to be related with more traffic accidents. It has been reported in the literature that this effect is often observed beyond the boundaries of the road network which had its speed limits changed. The term spillover effect expresses the tendency of the average speed of road networks with unchanged speed limits to be affected by the increase of av- erage speeds on adjacent road networks which experienced speed limit raises (e.g. Rock, 1995; Richter et al., 2004). This effect can be attributed to drivers’ attitude changes and speed acceptance that is transferred to other networks (Dutta and Noyce, 2005). This mechanism is illustrated in Figure 6.1. As a consequence, a speed limit increase on the motorway could lead to an increase in the number of accidents on the entire SRN and even beyond. However, as the extent of speed spillovers is not known, the potentially additional accidents on adjacent sections of the motorway cannot be quantified. Consequently, the estimated impact that will be presented here refers only to the accident increases on the motorway network, that is the minimum expected impact.

,

Figure 6.1: Graphical representation of the speed spillover mechanism on the SRN.

According to existing literature the average speed on a road is expected to be raised by 25% to 50% of the amount of the speed limit increase (e.g. Finch et al., 1994; Rock,

1995; Vadeby and Forsman, 2010). This means that if the speed limit of UK motorways increases from 70mph to 80mph (i.e. 10mph) the average motorway speed would be expected to increase by 2.5mph to 5mph. It is not clear how this change would affect the speed distribution of the networks. A speed limit change could cause a uniform shift to the speed distribution, or it could cause a more significant increase at higher speed conditions than at the lower ones. Considering that low speeds are normally caused by traffic congestion, the second case is more likely to be representative. Since it is not possible to predict the form of the new speed distribution though, the elasticity values that are presented here are estimated based on the expected changes on the average speed. The equation of the mean elasticity of the mth _{variable of the k}th _{accident category is:}

Elasticity= ∂E(y|xmk)

∂xmk

· xmk

y (6.1)

Table 6.1 shows the mean elasticity of accident with reference to speed and the estimated minimum and maximum percentage of increase for SV and MV motorway accidents based on the outcomes of theCondition-based SV KS-Sl (9)and theCondition-based MV KS-Sl (18) models respectively. As discussed, a 10 mph increase in the speed limit would result in 3.86% in average speed raise (i.e. the average 64.7mph speed would at least increase by 2.5 mph so the increase is ₆₄2._.5₇). Given that the mean elasticity of accidents with respect to speed is 2.595 for SV KS accidents (see Table 6.1), the corresponding increase in SV KS accidents would be at least 10.0% (i.e. 3.86·2.595). In a similar manner SV Sl and MV KS accidents would have a minimum increase of 6.14% (i.e. 3.86·1.591) and 3.57% (i.e. 3.86·0.925) respectively. The speed elasticity for the MV Sl accidents was chosen not to be presented here. As the relationship of speed with this accident type is negative, the elasticity of speed is a negative, too. Having no evidence to support that a speed limit increase can be associated with decrease in particular types of accidents and to keep the results conservative it is considered that the number of MV accidents that lead to slight injuries will not change.

Assuming that all other variables remain the same, SV KS are expected to increase by 10.0%-20.1% on motorways after one year of implementation of the speed limit increase. For SV Sl this number will fluctuate from 6.1% to 12.3%. This means that after a speed

increase of MV KS motorway accidents will be from 3.6%-7.1% equivalent to 11-21 more MV KS accidents. The overall predicted increase due to the anticipated average speed raise for all motorway KS and Sl accidents will reach 6.2%-12.1% (30-59 accidents) and 1.5% -2.9% (11-22 accidents) respectively. Considering the spillover effects this increase can be possibly even higher. The use of average elasticity may lead to underestimation of the impact because it does not take into account the unknown new speed distribution. However, these results provide clear evidence that a change on the 70mph current speed limit is expected to have a considerable and impact on road safety.

Table 6.1: Elasticity of speed and the minimum (mean speed increases by 2.5 mph) and maximum (mean speed increases by 5 mph) expected increase of motorway accidents by type.

Accident Expected Accident Increase (%) Additional Accidents

Type Elasticity Min. Max. Min. Max.

SV KS 2.595 10.027 20.054 19 37

SV Sl 1.591 6.141 12.283 54 108

MV KS* 0.925 3.571 7.141 11 22

*Estimation based on the average volume conditions (i.e. 148 vehicles per lane)

6.3

Recommendations for Accident Prevention Poli-