a weighbridge was installed in the spring of 1968 in the southbound carriageway of m.1 at friar's wash, hertfordshire, to measure the dynamic axle loads of moving vehicles, and was operated almost continuously for eighteen months. it was put into the nearside wheel track of the left-hand lane and the electronic apparatus recorded the number of axles carrying dynamic loads in increments of 1.81 mg (4000 lb) up to 16.33 mg (36,000 lb). analysis of the data obtained during 22 weeks spread over the 18-month period enabled estimates to be made of the minimum amount of recording necessary to obtain reliable information concerning the traffic and its damaging power. there was no marked seasonal variation in traffic, except during the summer holiday period and at christmas. on average, 128.630 axles per week travelled along the nearside lane and were equivalent in damaging power to 39,260 axles each carrying 8.16 mg (18,000 lb). although only 0.35 per cent of the axles carried dynamic loads exceeding 12.70 mg (28,000 lb), they contributed 10 per cent of the damaging power. the coefficient of variation of the weekly axle count was 5.5 per cent, but rose to 13.3 per cent when expressed as damaging power. there was a marked weekly pattern, the traffic at weekends consisting of fewer heavier vehicles and having a more variable damaging power. consequently, the damaging power of the weekly traffic could not be reliably estimated from recordings made on only one or two days over the weekend, but a reasonably reliable estimate was obtained from 5 1/2 times the damaging power of the traffic recorded on a monday. (a)

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