Community benefits from improved performance
Page 52
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RRL REPORT SHOWS HOW DELAYS TO OTHER TRAFFIC ARE REDUCED BY INCREASING THE POWER OUTPUT OF HEAVY ARTICS IN ASSESSING the benefits to the community through increasing the power-toweight ratio of goods vehicles, the author of the Road Research Laboratory Report LR 291, Mr. P. F. Everall, gives 114 pence per hour as the overall cost factor in running cars and light vans and 145 pence per hour as the figure for other goods vehicles. As mentioned in the summary of the report (CM December 12) the savings to the community of raising the power-to-weight ratio of a single articulated vehicle travelling 50,000km (31,000 miles) in 1969 are assessed to be £56 for an increase from 5 to 6 bhp/ton, £40 for 6 to 8 bhp/ton and £11 for 8 to 10 bhp/ton.
Three articulated vehicles were driven round the 990-mile (1,600km) cross-country course shown on the map, which has a total rural length of 620 miles (9981m) and an urban length of 370 miles (231km). The route test and a previous pilot study were based on the assumption that most delays which might be reduced by the introduction of minimum power-to-weight ratios occur on: 1, rising gradients and on the road distance travelled during the subsequent accelera tion period; 2, on a winding rural road, where a queue has built up (as a result, for example, of bunching on a gradient) and vehicles cannot overtake; 3, in situa tions where improved accelerations would reduce delays, for example in an urban area or after the vehicle has passed an .obstruction such as a road works or a parked vehicle.
Outfits employed on the tests comprised one hauled by an Atkinson tractive unit powered by a Gardner 150 bhp diesel that grossed at 27.9 tons and had a power-toweight ratio of 5.36 bhp/ton; one having an Atkinson tractive unit with a Gardner 180 bhp engine running at 27 tons gross which had a power-to-weight ratio of 6.67; and one with an AEC Mandator unit of 226 bhp operating at 23.2 tons gross that provided a ratio of 9.68 bhp/ton.
A Leyland Beaver powered by a turbo charged diesel and equipped with a semiautomatic gearbox was used in supplementary tests—it had a gross weight of 29.65 tons and a ratio of 8.09 bhp/ton. Only part of the 990-mile route was covered by the Leyland Beaver but the results show that the delays it caused were 25/50 per cent of those that a conventional unit of the same power-toweight ratio would create. The diesel engine of this unit develops a maximum torque of 650 lb.ft. at 1,400 rpm compared with 485 lb.ft. at 1,100 rpm, 536 lb.ft. at 1,100 rpm and 618 lb.ft. at 1,500 rpm respectively of the other units.
The Beaver was fitted with a five-speed overdrive-top gearbox, whereas the gearboxes of the three main test vehicles were equipped with six-speed overdrive-top gearboxes.
Vehicles that had to queue behind the test' lorries were classified in groups and comprised 1, cars and light vans following closely behind the lorry; 2, other lorries and buses; and 3, cars and light vans that were behind lorries and buses following the test vehicle. It was assumed that the speeds at which the other vehicles would have travelled in the absence of the test vehicle corresponded to the speed of an undelayed journey for cars and to the speed of the test vehicle with a power-to-weight ratio of 9.68 in the case of lorries and buses.
Pilot study In the pilot study, runs were made over a rural trunk road, A423, from Henley to A404 near Maidenhead, which has a number of gradients (in both directions) and carries fairly heavy traffic.
Extra runs were made on a winding hill on A423 to enable data to be compiled on speed variation and delays according to the powerto-weight ratio, and acceleration rates were measured.
A route for the main test was chosen that had no motorways or long lengths of dual carriageway, as trunk and Class 1 roads carry most of the heavy vehicle traffic. Discounting the distances travelled on shorter lengths of dual carriageway reduced the trunk road mileage (for assessment purposes) from 61 per cent to 54 per cent of the total distance covered. Gradients of more than 2 per cent accounted for 196 miles (315km), the maximum gradients being less than 8 per cent, apart from about 3.7 miles (6km) of steeper gradients.
The three vehicles were dispatched at fiveminute intervals from stage points on the route approximately 49.5 miles (80km) apart and the time interval at intermediate points rarely exceeded 20min. The vehicles were handled by experienced artic drivers with an observer in the cab. Delays to cars and commercial vehicles were measured over 55 miles (88.3km) of rising gradients of the test route to show the gains derived from increasing the bhp/ton of the vehicles. In the case of cars, the increase from 5 bhp/ton to 6 bhp/ton reduced delays from 105.5 minutes to 69.5 mm (36 min) while improving the ratio from 6 bhp/ton to 8 bhp/ton and from this ratio to 10 bhp/ton reduced delays from 69.5 min to 35.5 mm (34 min) and from 35.5 min to 17.5 min (18 min) respectively. The reductions of delays to commercial vehicles were from 34.5 min to 8 InM (26.5 min), from 8 min to 1.7 min (6.3 min) and from 1.7 min to° min (1.7 min) respectively. It is noted in the report that the delays per km on rising gradients varied from 5 to 14 times the delays on the whole route.
After stating that power-to-weight ratio was a suitable guide to the performance of vehicles on rising gradients, the author observes that due regard should, however, be paid to other factors such as the transmission.
Special reference is made in the report to the performance of the Leyland Beaver on a winding hill, and its relatively high average speed on the hill (with a consequent reduction in delays) is attributed to the elimination of wasted time in gear changing provided by the semi-automatic gearbox. Its speed was 16.2 mph (26 km/h) compared with a speed of 12.4 mph (20 km/h) that the best of the other test vehicles would have averaged.