The buses in San Diego are not very good.

The busiest urban lines average about 10 mph. The 215 Rapid bus, which runs from SDSU to downtown mostly along El Cajon Boulevard and is intended to be faster and serve fewer stops than a regular bus, falls short of true bus rapid transit standards, and averages only 12 mph. It’s not much faster than a cyclist.

So how can San Diego improve its bus network, so people who rely on it get around faster?

Transportation consultant Jarrett Walker has recommended cities around the world lay out their bus network as a simple grid to maximize its usefulness. It allows passengers to get from anywhere, to anywhere, with just one transfer. If the buses are frequent enough, the extra time passengers spend waiting for the connecting bus is not onerous. West Coast cities with effective bus systems utilizing a grid include Los Angeles, Portland, Ore. and Vancouver, Canada.

San Diego does not really have a grid. Instead, it has multiple grids, interrupted by freeways, canyons, rivers and Balboa Park. Major thoroughfares in the urban core deal with constant interruptions. For instance, 6th Street is the last north-south arterial street – or high-volume urban road – out of downtown before Balboa Park interrupts the grid. The next north-south arterial to its east is 30th Street, more than a mile and a half away. But farther north, 30th Street does not extend past Interstate 8, where Mission Valley interrupts the traffic pattern. Texas Street connects the University Heights area with Mission Valley, but it too ends at Balboa Park.

In National City and Chula Vista, MTS’s bus system relies on a more continuous grid. There are freeways and the Sweetwater River, but the arterial streets cross all of them, unlike in San Diego’s urban core. The second busiest bus in the county, however, the 929, transitions from Third Avenue in Chula Vista to Fourth and Highland Avenue in National City, before swerving to serve downtown San Diego. That bus cannot continue north on the same street in San Diego, 43rd, because it is interrupted by Imperial Marketplace and Greenwood Cemetery.

Thanks to these physical and land-use related barriers, even the buses that do exist cannot perform very well. Cars can avoid grid interruptions by using freeways or turning to a different arterial street, but buses cannot do so as easily. Freeways have no room for local stations, so freeway buses and local buses serve separate travel markets. And turning to a better arterial, such as from 30th to Texas, is easier for a car than for a bus: Buses have to slow down when they turn, and can miss lights more easily if the stoplights are timed for cars.

Even a bus with a continuous arterial in a city is unlikely to be reliable if it faces regular interruptions.

Uninterrupted grids, Walker said, are especially valuable because they can distribute traffic across many parallel streets. In Vancouver, a potential peer city San Diego’s MTS could use as a benchmark, there are no freeways within city limits, and there are many grid arterials for drivers, which are continuous for many miles. Within three-quarters of a mile Vancouver has four east-west streets — 4th Avenue, Broadway, 12th and 16th. The limited-stop buses average 18 mph on 4th and 12th on Broadway, and generally arrive on schedule.

But given its current infrastructure, San Diego cannot hope to emulate Vancouver. Even when there are arterial streets at the right place, the road network dumps all through-traffic on them, rather than dispersing it along a series of parallel streets. Most local buses average around 10 mph, but the segment of the well-used 929 that runs continuously on Highland and 4th, crossing Sweetwater River, only averages 8 mph.

This congestion makes it impossible to run buses reliably.

San Diego isn’t alone. In San Jose, a bus network redesign by Walker’s consultancy is creating many routes that come scheduled to arrive every 15 minutes — but those routes cannot stick to their schedule, because of freeway grid interruptions. A BART extension into San Jose, due to open next year, is also scheduled to run trains every 15 minutes, but the bus-BART transfers will not be timed, because the buses are not reliable enough to make the connection.

Grid connectivity is even poorer in San Diego than in San Jose. That means San Diego needs to look to models outside the midwestern and western United States, where most cities have relentless grids. In the United States, the least gridded major city is Boston. In Boston, buses use arterial streets where they exist, but those are usually short. With slow traffic, the main form of public transit in Boston is the subway. Most buses are subway feeders; they don’t even enter downtown, due in part to its narrow streets. One possible lesson for San Diego, then, is to look to the trolley as its primary form of public transit, with most buses – with the exception of the University and El Cajon corridors –useful primarily as trolley feeders.

Bay Area public transportation activist Elizabeth Alexis, of Californians Advocating Responsible Rail Design, suggests the Zurich model as a way of improving public transit. In Zurich, the emphasis is on keeping to the timetable. When service is reliable, planners can schedule buses and trains with fast connections, so a trip involving multiple transfers may not be any slower than a one-seat ride. In San Diego, this would have implications for transit service in the suburbs around key Green and Orange Line trolley stations. (The Blue Line comes every seven to eight minutes, frequent enough that timed connections have little value, but the other two lines come every 15.)

Timed transfers, however, require more reliability than San Diego’s bus network has today. Achieving this reliability would require giving buses priority in traffic. That means dedicated lanes, especially on the most congested streets, and signal priority at intersections. Neither dedicated lanes nor signal priority is expensive, but both are politically contentious, since they involve taking space away from cars and giving it to buses.

As a solution, Alexis advocates for a way to bypass controversy: Install bus-only lanes on streets that are not yet congested, but that the planners believe will become congested in the future. This could be viable in growing suburbs served by the North County Transit District’s Coaster and Sprinter. A tough political fight is probably unavoidable in San Diego proper.

A less controversial way to both speed up buses and make them more reliable is off-board fare collection, the way ticketing works on the trolley. Though many American cities, such as New York, consider this a specific feature of bus rapid transit, San Francisco recently implemented it systemwide, allowing passengers to board the bus from any door. This has made boarding times both faster and more consistent. It could be an invaluable component of a bus plan for San Diego that emphasizes reliable service.

The street layout in San Diego is unusual in North America, with so many grid interruptions. This means that copy-and-pasted solutions from peer cities up the West Coast may not work as well as they did in their original setting. San Diego’s transit system probably needs to be rail-centric, with buses feeding rail from outlying areas. But the buses can’t just run in shared lanes on a grid, as is the case in many U.S. cities. They need priority on lanes and at intersections, and all-door boarding, to become as reliable as they need to be for a modern transit system.

Alon Levy is a Paris-based mathematician and public transportation policy writer.