WASHINGTON — Pedestrian deaths are climbing faster than motorist fatalities, reaching nearly 6,000 deaths last year — the highest total in more than two decades, according to an analysis of preliminary state data released Thursday. Increased driving due to an improved economy, lower gas prices and more walking for exercise and environmental factors are some of the likely reasons behind the estimated 11 percent spike in pedestrian fatalities in 2016. The figures were prepared for the Governors Highway Safety Association, which represents state highway safety offices.