Drivers on Darnestown Road and Montrose Road will see lower speed limits posted by the end of August as Montgomery County and the Maryland State Highway Administration finish reducing speeds on five jointly maintained corridors.

The county's Department of Transportation has already lowered speed limits on 33 streets, mostly residential, since July 2025, officials announced Monday. The five state-county corridors also include Josiah Henson Parkway in North Bethesda.

New limits on those roads will range from 25 to 35 mph depending on classification: Urban Core roads will be posted at 25 mph, Urban Center roads at 30 mph, and Suburban Activity Center roads at 35 mph, according to Montgomery Community Media.

The Darnestown Road segment being reduced is described in county materials as being in the Rockville area, though the road extends west through Potomac. The county has not specified which stretch will get new signage.

The reductions stem from Maryland House Bill 562, passed in 2021, which authorized Montgomery County to set speed limits as low as 20 mph on local roads and 15 mph in rare cases. Since the law took effect, more than 250 neighborhood streets countywide have had their limits reduced.

Most of those changes came at residents' request. Vision Zero coordinator Wade Holland told WTOP on Monday that speed limits are set after engineers analyze a road's location and design.

"When you get to that 35 to 40 mph speed limit, if you strike a pedestrian or cyclist, that pedestrian or cyclist has a 50/50 chance of surviving," Holland said.

Maryland Highway Safety Office data cited by Montgomery Community Media puts it another way: a pedestrian hit at 30 mph faces a 40% chance of serious or fatal injury, but at 20 mph that risk drops to 10%.

The speed limit work is part of the county's broader Vision Zero initiative, which aims to eliminate serious and fatal traffic crashes by 2030. Serious and fatal crashes on Montgomery County's High Injury Network corridors fell 29% in fiscal year 2025 compared to the pre-pandemic years of 2015 through 2019, according to the county's annual Vision Zero report released in November 2025.

County Executive Marc Elrich said in a July 20 statement that reducing speed by just 5 mph "can prevent an unimaginable tragedy." MCDOT Director Chris Conklin said engineers conduct safety, engineering, and traffic analyses before implementing any reduction, adding that lower speeds are especially important in areas with heavy pedestrian and bicycle traffic.

A map of all roads with reduced speed limits is available on the Montgomery County GeoHub at geohub.montgomerycountymd.gov.

Holland said the county is also redesigning high-crash corridors beyond the speed limit changes. New Hampshire Avenue near the Beltway is under construction, and work on Montrose Road, University Boulevard West, Montgomery Village Avenue and Randolph Road is planned for later in 2026.