In 2010, the then Minister for Transport (Hon Steven Joyce), assisted by the Associate Minister of Transport (Hon Nathan Guy), launched the Safer Journeys strategy, which identified the vision of a New Zealand road system increasingly free of death and serious injury. In 2014, 295 people were killed and 2,709 people had hospital stays as a result of road traffic injury. The social cost of crashes on New Zealand’s roads in 2013 was $3.12 billion. Regardless of what causes a crash, speed determines the impact of the crash and the severity of the injury. The relationship between speed and travel time is an important part of the conversation on speed because decisions to speed may be based on the desire to save time. For example, a survey of New Zealand drivers found one-third agreed when they speed it is because they want to get where they are going sooner. The theoretical relationship between mean travel speed and travel time is straightforward. The effect of a reduced maximum speed limit on mean speed, travel time and fuel consumption in real driving conditions is less clear. The New Zealand road network differs from those in many other countries, for example in the number of motorways, tortuous sections and hills. Drivers travel at the highest speeds in open road situations, but speed choice is also important in urban environments where there are frequent intersections and hazards. Half of the deaths and serious injuries from road crashes occur in towns and usually at intersections. Data collection method: This project investigated the effect of setting different maximum speed limits on mean speed, actual fuel consumption and travel time for six different New Zealand routes in real driving conditions. The six selected routes included three short urban routes and three long routes chosen to be representative of metro driving or open road driving across a variety of road classifications. The short urban routes, one in Auckland (12km) and two in Wellington (10km and 6km respectively) were driven at 40km/h and 50km/h for between 102 and 120 times. The long routes, Auckland to Tauranga (211km), Hastings to Levin (197km) and Christchurch to Kaikoura (178km), were driven at 80km/h, 90km/h and 100km/h between 40 and 42 times. Trips were only included in the final dataset where the drivers stayed below the trip maximum speed limit plus a 2km/h margin of error for 95% of the driving time excluding idle periods. One driver was employed for each route. All routes were driven in different vehicles of the same model, 2013 Toyota Corollas, with data loggers that connected to their vehicles. Findings 1 Mean speed decreased when the maximum speed limit was decreased, but by a smaller proportion. Decreasing the maximum speed on the long routes by 20% from 100km/h to 80km/h decreased mean speed by between 8% and 12%. The same percentage decrease in maximum speed on the short routes (20%, from 50km/h to 40km/h) decreased the mean speed by 9% on Wellington short route one, 7% on Wellington short route two, and 14% in Auckland. Drivers’ speed is not just dictated by the maximum limit in real-world driving. This difference in the size of the decrease in maximum and mean speeds is likely to be explained by factors such as traffic, road conditions and intersections that prevented the drivers travelling at the maximum trip speed. 2 Travel time: Decreasing maximum speed had a strong effect on travel time. Travel time increased by a smaller proportion than the decrease in maximum speed. Results across all of the routes tested in this study consistently demonstrated that decreasing maximum trip speed increased travel time. However, the extent of the increase in travel time varied. On the long routes drivers spent much of their driving time at cruising speeds close to the maximum speed, but travel time still increased by a smaller proportion than the reduction in the trip maximum speed. The potential effects of other factors (traffic and weather) on speed and travel time were explored but most of the travel time variation was explained by the change in maximum speed. On the short routes and two of the three long routes, idle time and travel time were strongly correlated. However, there was no significant difference in the amount of idle time when comparing the tested speeds. Taken together, the results show that trip maximum speed limits have a strong effect on travel time; however, increase in travel time is not equally proportional to the decrease in travel speed. Other factors, such as traffic volumes and number of controlled intersections, affect travel time in urban routes to a greater extent than long distance trips. The evidence for a strong and consistent relationship between travel time and maximum trip speed on long routes in real driving conditions provides useful information for understanding the effect that speed limit modifications may have on drivers’ fuel consumption and travel time. 3 Fuel consumption was closely related to maximum speed, with higher maximum speeds leading to higher fuel consumption on all but one route. Lower maximum speed decreased fuel consumption by a smaller proportion than the decrease in mean speed on two of the short routes. However, the decrease in fuel consumption was larger than the decrease in mean speed on the long routes. Fuel consumption and maximum speed were highly correlated with on all three of the long routes. Maximum speed was a significant predictor of increased fuel consumption on all three long routes. Higher ratings for the effect of weather predicted increased fuel consumption on two of the three long routes and idle time predicted higher fuel consumption on one route. The findings suggest the effect of factors such as weather and traffic may be more substantial if they were recorded with more sensitivity. However, it is clear that adopting the higher speeds had a strong and consistent effect on fuel consumption. On the short routes, there was less consistency. Five percent and 3% decreases were identified on the Wellington short routes. Pulling over to allow following traffic to pass at the lower speed could have increased fuel consumption, closing the gap between the lower speed and the higher speed. As with travel time, factors other than maximum speed had a greater effect on the short routes than on the long routes. Traffic density, traffic light stops and idle time were important variables in understanding fuel consumption. This is consistent with evidence in the literature that ‘stop-and-go’ driving, where the driver is frequently decelerating at delay points and subsequently accelerating, can result in higher fuel consumption. Higher trip maximum speed predicted increased fuel consumption on the two Wellington short routes but not the Auckland short route. The Auckland short route in particular was dominated by factors causing the driver to come to a complete stop such as traffic lights and pulling over for other drivers. Increased idle time predicted higher fuel consumption on all three short routes. Though it was an important factor in predicting fuel consumption, there was no significant difference in the idle time recorded between the different maximum speeds. 4 Variation within and between routes: The routes had different characteristics and drivers encountered different challenges, leading There was more variation recorded in fuel consumption and travel time on the short routes than the long routes. On the short routes, idle time and traffic ratings were significant predictors of fuel consumption, highlighting the influence of factors other than trip maximum speed. The direction of travel was also an important variable. That these factors were significant emphasised the effect that small variations in the driver’s experience on each trip had on fuel consumption and travel time. Short delays, such as those caused by a traffic light or another driver parking, could have relatively large effects on the measurements for those trips. Differences across routes may be attributable to factors which were not controlled by the method adopted for this project. While the vehicles were of the same model, driver technique can play a role in the amount of fuel consumed and travel time. As the drivers were different for each route, differences in the figures for fuel consumption and travel time savings between routes could be a result of differences between the drivers. Driving style can account for large variance in fuel consumption. There was also variation in the type of terrain covered by each of the routes. All three long routes had sections through the outskirts of major cities and sections where the nature of the road limited speed. 5 Safety: Speed is an important factor in all crashes in both urban and open road environments. Any discussion of different travel speeds and their time and fuel costs must acknowledge the wellestablished relationship between travel speed and safety. The relationship has two important aspects: the effect of speed on the risk of crashing and the effect of increased speed on the severity of a crash should one occur. Increasing speed increases both the risk of crashing and the severity of crashes should they occur. The relationship between speed and safety is important in both urban environments and open road environments. Conclusion: The results of this study demonstrate that decreasing maximum travel speed results in decreases in mean speed but by a smaller proportion than may be expected. Decreasing maximum speed increases travel time but by a smaller proportion. Fuel consumption decreased on five of the six tested routes as the maximum speed was reduced. The proportion by which fuel consumption decreased was less than the proportionate decreases in mean speed on two of the three short, urban routes but was greater than the proportionate decreases in mean speed on all three long routes. This information will inform the conversation on the costs and benefits of different speed limits. It is, however, important to note that any conversation about the costs and benefits of different speeds on travel time and fuel consumption takes place in the context of the well understood safety consequences of increased speed. Speed plays a part in every crash. Speed determines the impact of the crash and the severity of the injury. This project provided evidence that for six different New Zealand routes in real driving conditions the benefits of reducing fuel consumption and the costs of additional travel time as a result of decreasing maximum speed were less than expected based on theoretical prediction of travel time. (Author/publisher)
Samenvatting