Major thoroughfares that serve transit connecting the city’s downtown and neighborhoods may have significant imbalance towards motor vehicle capacity that is poorly used at most times of day, creating daunting or intolerable walking and bicycling conditions, and broad swaths of impermeable surface that exacerbate stormwater runoff and outfall events. Auto-oriented land uses along major streets create polluted runoff, making water quality treatment a priority.
These wide streets are often designed for peak hour automobile volume, and are underutilized throughout the rest of the day and have not been optimized for transit. Over-provision of motor vehicle space results in frequent speeding and poor walking and biking conditions, degrading the public realm and depressing adjacent property values and land uses that would generate activity.
Moderate to heavy turn volumes and an overly high design speed result in long turn bays and wide curb radii, covering large areas of the right-of-way with impermeable surfaces. Left turns across multiple lanes introduce high-risk conflicts between motor vehicles and other street users.
High-capacity transit can move more people in less space, unlocking street space for GSI and improving environmental performance. Shifting trips from cars to transit and lowering motor vehicle speeds has the effect of decreasing emissions, surface pollutants, and noise impacts while increasing safety. Use newly identified space, including medians, curb extensions, and bikeway buffers, to provide high-capacity stormwater facilities.
Dedicated right-of-way for high-capacity travel modes may itself be integrated with stormwater facilities; rail guideways or concrete busways involve large continuous linear spaces, which can be outfitted to infiltrate stormwater without interrupting accessible paths. As illustrated, linear bioretention planters capture stormwater as sheet flow (unchannelized runoff) from the transitway. Curbs are not necessary alongside fixed guideways, but are recommended next to motor vehicle or bicycle travel lanes to reduce incursion.
Wide rights-of-way with less dense underground utilities sometimes present greater opportunities for stormwater infiltration than smaller streets. Long blocks with few curbside access needs and wide planting strips may allow for implementation of large swales with significant infiltration capacity. Plantings and trees not only improve stormwater performance, but also help to transform auto-oriented streets into places for people, absorbing noise and air pollution and introducing valuable habitat into a corridor.
The right turn lane has been replaced with a green refuge island, and turn speeds have been reduced with a tighter curb raidus. The left turn has been restricted or given a signal phase to improve safety and transit reliability. The left turn lane has been closed and repurposed as part of the green median, with a pedestrian refuge island.
The slip lane, which had enabled unsafe high-speed turns, is closed and its footprint reallocated to a large bioretention planter. A pedestrian walkway is provided along the diagonal desire line, creating an opportunity for informational signage and placemaking.
As speeds decrease, the risk of vehicles entering stormwater facilities also decreases. Reduce target speed to safe and appropriate urban speeds—typically 20–25 mph, and rarely more than 30 mph—through the reassignment and narrowing of motor vehicle traffic lanes; a transit-supportive signal progression speed, usually below 20 mph; and traffic calming elements. High-visibility or retro-reflective vertical elements may be placed at the leading edge of GSI facilities to reduce the risk of motor vehicle incursion, especially where car speeds remain high.