题目选项分析

A recent spate of road traffic accidents has stirred up much controversy over ways to increase road safety, and though some protective measures, like the establishment of pedestrian subways and vehicle flyovers, having been taken decades ago, great increases in vehicle density and in pedestrian population are levying a heavy burden on the traffic system.

A recent spate of road traffic accidents has stirred up much controversy over ways to increase road safety, and though some protective measures, such as the establishment of pedestrian subways and vehicle flyovers, were taken decades ago, great increases in vehicle density and in pedestrian population are levying a heavy burden on the traffic system.