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Speed lumps, roundabouts, diverging diamonds & more
union road 2
Photo by Wayne Thallander Northbound traffic stops at a signal as southbound traffic moves back to the west side of Union Road on Manteca’s new diverging diamond interchange.

It’s ironic that the No. 1 complaint people have about roundabouts are the fact they slow traffic down.

That is one of their design functions. Others are to keep traffic flowing, reduce accidents, make it safer for pedestrians to cross and — in air quality conscious California — reduce pollution from idling vehicles when compared to the other alternatives of stop signs or traffic signals.

Roundabouts are tools the City of Manteca has been turning to in recent years to slow traffic down and improve safety as the city grows.

The city has deployed them due to the compliant that elected council members keep hearing more than others — speeding.

The past 14 months saw two new tricks deployed on city streets that effectively slow vehicles and enhance safety. They were speed lumps and California’s first diverging diamond interchange at Union Road and the 120 Bypass.

The interchange by crossing Union Road traffic back and forth on the bridge deck takes the number of conflict points — primarily left turns — with other vehicles from 26 down to 14. The cross-crossing of the travel lane also slows Union Road traffic down. But the elimination of a series of signal movements to make left turns into and off ramps means traffic movements aren’t interrupted and therefore increases the volume of traffic the interchange handles.

The fiscal appeal lies in the fact the design compared to a traditional interchange takes less land and can use existing bridge decks. That trims the cost by almost $10 million while reducing the construction timeline significantly.

When the time comes to upgrade the 120 Bypass interchanges at Airport Way and Main Street, city officials have indicated they will likely also be the more cost effective and safer diverging diamond design.


Councilman Halford would like

to see more speed lumps

While the diverging diamond is high profile, the solution that has effectively reduced speeding on one residential street is not.

Speed lumps were placed on Hacienda Avenue midway between Orchard Way and La Mesa Way on a straight section between Louise Avenue and Alameda Street. They were installed after residents along the street — frustrated at increased use of Hacienda as a shortcut by drivers that routinely ignored the residential speed limit — accessed the City of Manteca’s traffic calming program. They got the ball rolling by following protocols to determine if neighborhood support met pre-established thresholds. Once that was established, the city went ahead and installed the speed lumps.

The speed lump is a variation on the speed hump. It adds two wheel cut-outs designed to allow large vehicles, such as emergency vehicles and buses, to pass with minimal slowing. The design limits passenger cars and mid-size SUVs from fully passing through the cutouts, but allows one set of wheels to pass through the cut-out while the other set is required to travel over the lump.

Speed lumps have a similar reduction in speeds when compared to speed humps.

The speed lumps are within two blocks of Councilman Charlie Halford’s home.

“The city should have been installing speed lumps sooner,” Halford said.

The recently elected Halford is the council’s “traffic expert” given three decades of patrolling city streets including a stint as police chief before retiring.

Halford noted they are effective at slowing down traffic on long, straight, and wide residential streets where most drivers feel it is safer to go faster despite there there being children, bicyclists and people backing out of driveways.

On Tuesday during  an 8-minute period, 17 vehicles passed over the speed lump including a FedEx delivery van. All slowed down crossing the lumps without making additional traffic noise as would happen if they crossed them too faster. Only one noticeably accelerated after crossing. There was an 18th vehicle — a Manteca Police motorcycle officer — that passed between the lumps.


Bike lanes do double

duty for traffic calming

The lumps aren’t allowed for use on arterials or connector streets such as Mission Ridge Drive and Moffat Boulevard.  In both cases the city repeated the success they have enjoyed on South Powers Avenue by narrowing the travel lane by the placement of bicycle lanes.

Mission Ridge Drive also got the city’s first permanent solar powered traffic speed radar sign that flashes a vehicle’s speed when it surpasses the posted speed limit that is delineated on a traditional sign mounted below it.

Several Mission Ridge Drive residents contacted this week said they have noticed a decrease in speeding. One indicated they would still like to see stop signs installed at the street’s T-intersection with Locust Avenue.

The bike lanes on Moffat have accomplished two things. They have slowed down speed somewhat on the mile stretch between Spreckels Avenue and Main Street that is wide, smooth and has minimal use of in-street parking. They also have made it somewhat easier for pedestrians to cross especially at Cowell Avenue where the re-striping of the center lane in conjunction with the bike lanes moved vehicles away from the curb and reduced sight-line vision issues with fencing and parked semi-trucks

Halford said the striping works to slow most motorists down as it visually narrows the travel lane.


Manteca needs to

do a better job

The retired police chief noted it take a three-prong effort to address speeding — education, enforcement, and engineering.

“We need to do a better job at all three,” Halford said.

While all three work in conjunction, the most effective may just be engineering that employs passive designs such as roundabouts, narrower streets, and the bulb-outs along with roundabouts that are now common at parks in new neighborhoods that border intersections with at least one street that functions as a collector street.

Halford, like his fellow council members, would like to see the police department traffic enforcement unit beefed up. That said, Halford noted passive speed control measures are essential.

“You obviously can’t have police officers 24/7 everywhere,” Halford pointed out.


It is why newer developments are being approved with a liberal sprinkling of roundabouts on access streets as well as near parks and future school sites. And where there are “undeveloped” areas where new intersections are being created on major arterials, larger roundabouts have also been required.

Research by the Institute of Highway Safety as well as the Federal Highway Administration shows that roundabouts used at intersections that once had — or would have had — stop signs or traffic signals revealed roundabouts:

*reduce overall collisions by 37 percent.

*reduce injury accidents by 75 percent.

*reduce fatal accidents by 90 percent.

*reduce pedestrian versus vehicle accidents by 40 percent.

The design of roundabouts reduces speed by 15 to 20 miles per hour. Given all turns at the intersection are in the same direction, half of the potential conflicts movements are eliminated as are the potential for T-bone and head-on collisions that have a significantly higher rate of serious injuries and even death.

The most high profile roundabout on the books is the one at the future intersection of South Main and Raymus Parkway required of developers of the 1,301-home Griffin Park neighborhood that is nearing groundbreaking.

Both South Main and Raymus Parkway will be wide enough to each accommodate four lanes of traffic.

As such, the roundabout will be the first in Manteca where two major arterials cross.

Although roundabouts have superior safety statistics, eliminate red light running, are less expensive in the long-haul to maintain, enhance pedestrian safety, improve air quality, and keep traffic moving they are not in the cards for almost all existing intersections.

That’s because the most optimum design requires using land on all four corners of an intersection. None of the land around the future Raymus Parkway-South Main intersection s developed.

The city a few years ago weighed going with roundabouts instead of traffic signals at some point in the future along Woodward Avenue with its intersections at Main Street and Union Road. In both cases municipal staff determined the most effective roundabout design could not be used without removing improvements that consist primarily of yards and sound walls. As a result, plans currently call for traffic signals to be installed at some point in the future.


To contact Dennis Wyatt, email