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A new report shows that speed humps are changing the way people drive on two streets in the Point Dume neighborhood.
Speed humps, part of a family of vertical traffic calming measures that includes, according to Caltrans, speed bumps, speed lumps, speed tables, speed cushions and textured pavements, have been around the point for about five years.
But with motorists driving too fast on Dume Drive and Fernhill Drive, where the speed limit is 30 mph and 25 mph, respectively, some residents more recently called for their own speed humps.
Over objections from some in the areas, the city responded by installing speed advisory signs and speed humps on both streets, with the last of the humps installed in December 2020.
Unlike horizontal measures such as roundabouts or serpentine streets which impede straight-through forward motion, speed humps and the like use forces of acceleration to discourage speeding.
According to data from the Traffic Logix advisory signs, the total daily vehicle count on Dume Drive is between about 4,500 and 6,500. Of those trips, the average number of speed violations before the speed humps were installed ranged from 328 on Sunday to 431 on Friday, according to a study in February-March 2020, with the worst days, in ascending order, being Wednesday, Thursday and Friday.
During that same period this year, data shows in some cases a nearly tenfold drop in speed violations, with Thursday and Saturday topping the list at 57, while Friday was next with 50.
Monday saw the fewest violations this year with 31.
Turning to Fernhill Drive, last year’s total vehicle count ranged from 2,211 (Monday) to 3,352 (Saturday). This year, Thursday was the lowest with 3,031, while Sunday had the highest count, 4,204.
Speed violations on Fernhill were typically lower than Dume last year, with an average of 261 on Wednesday, 313 on Sunday.
Interestingly, the change in violations after the speed humps were installed was much less dramatic — and still every day clocked many more violations than on Dume. Wednesday saw the fewest, with 66, while Saturday had the most speed violations on average at 102.
The big question is how did the speed humps affect the average speed?
Traffic engineering generally takes into account what’s known as the 85th percentile speed, that is the speed at or below which 85 percent of all vehicles travel under normal conditions past some sort of monitor.
According to the city’s data, the average speed on Dume Drive before speed humps was 26 mph, with 32 being the 85th percentile speed.
After the humps were installed, the average speed dropped to 21, while the 85th percentile speed was 27.
On Fernhill Drive, the pre-humps average and 85th percentile speeds were 22 and 29, respectively; with the humps in place, those speeds were 19 and 24, respectively.
According to the report, “it appears the newly installed speed humps have made a significant impact in reducing speed on Dume and Fernhill” drives.