This is an experiment which has been conducted many times in different places around the world, with strikingly similar results. The US was probably first, driven both by a desire to reduce lead exposure, but also because lead poisons the catalytic converters which were required to reduce CO and NOx emissions. There are many studies documenting the results at different times, in different places, but here are four examples:
The Journal of the American Medical Association published an article showing a 78% drop in blood levels among Americans from 1976 to 1991. Here is the abstract, access to the full paper requires payment. The authors attribute the drop primarily to the removal of lead from gasoline. From the abstract:
The mean blood lead level of persons aged 1 to 74 years dropped 78%, from 0.62 to 0.14 μmol/L (12.8 to 2.8 μg/dL).
Conclusions. —The results demonstrate a substantial decline in blood lead levels of the entire US population and within selected subgroups of the population. The major cause of the observed decline in blood lead levels is most likely the removal of 99.8% of lead from gasoline and the removal of lead from soldered cans. Although these data indicate major progress in reducing lead exposure, they also show that the same sociodemographic factors continue to be associated with higher blood lead levels, including younger age, male sex, non-Hispanic black race/ ethnicity, and low income level. Future efforts to remove other lead sources (eg, paint, dust, and soil) are needed but will be more difficult than removing lead from gasoline and soldered cans.(JAMA. 1994;272:284-291)
Here's an article that's not on a pay site which cites the same information, and describes a very strong correlation between the dropping blood lead levels and the reduction in use of leaded gasoline. It states
The phaseout had striking results in the U.S. Before it took place, 88 percent of children had blood lead levels higher than 10 mcg/dl. Afterwards, only 9 percent had elevated blood lead levels. The blood lead levels of all Americans declined 78 percent between 1978 and 1991, falling in exact proportion to the declining levels of lead in the overall gasoline supply.
Abstract This study was conducted to relate blood lead levels in infants to changes in lead emissions in Santiago, Chile, a heavily polluted setting where leaded gasoline began to be replaced with unleaded gasoline in 1993. Over an 18-mo period, 422 infants had blood lead levels, cotinine, and iron status determined at 12 mo. Blood lead levels fell at an average rate of 0.5 microg/dl every 2 mo, from 8.3 to 5.9 microg/dl, as the city experienced a net fall of 30% in the quantity of leaded gasoline sold. Time progression, car ownership, serum cotinine, and type of housing were significantly associated with a blood lead level > or = 10 microg/dl. In this study, the authors demonstrated that infant blood lead levels, even if relatively low, can drop very rapidly in conjunction with decreases in environmental lead exposure.
Totally 4,967 samples in target areas were used to review the trend of blood lead level in this paper. Average lead concentrations by areas were from 15.2 to 21.0 ug/dl in 1981 and 22.3 to 34.3 ug/dl in 1988, but were 8.8 to 11.1 ug/dl in 1992 and 4.4 to 4.8 ug/dl in 1995. On the other hand, the consumption of leaded gasoline was at a peak in 1988. Blood lead level showed a very close relationship with the consumption of leaded gasoline in the change pattern (p < 0.01) and showed a rapid declining trend since the use of unleaded gasoline, especially from 1988 when Seoul held the olympic games. For example, the blood lead levels were 15.2 ug/dl in 1981, 20.2 ug/dl in 1985, 24.3 ug/dl in 1988 and 3.9 ug/dl in 1993 in Yochon area