This is the soundtrack of a person walking across a bridge above a busy street. The first sound we hear is the Doppler effect of a car approaching, then disappearing beneath the bridge, making a noise roughly like "eeee-UURRHH!!" A moment later, the same car emerges from beneath the bridge. "WHIFFF-urrrrh!" In addition to the sound, the motion of the speeding car seems to create a vacuum in its wake. Nearly stationary in comparison, the pedestrian has the sensation that he is at risk of being sucked off the bridge by the wake of the departing car and falling into the busy roadway below. That the danger is illusory does not make the experience more pleasant. A second, and possibly greater, problem is the sheer awkwardness of using pedestrian bridges. In most cases, pedestrians must mount a set of stairs, or an exceedingly long spiral ramp, then walk across the bridge ("eeee-UURRHH!!" "WHIFF-urrrrh!"), and finally walk down a second set of stairs or a ramp on the opposite end. These issues help explain why pedestrian bridges have failed to gain popularity. Although it is understandable that bridges would be attractive in concept as a way of creating convenient footpaths across wide and dangerous thoroughfares, they rarely work in urban planning. A downtown area might be able to span a river or a valley, but a bridge over a busy street might strike planners as a bridge too far. For that reason, I confess to some initial skepticism when I heard that the City Council of Yorba Linda, a city of 60,000 people in north Orange County, recently approved a new master plan for the city‚s downtown area with a bridge as a central feature. The basic goal of the plan is to bring visibility and new pedestrian activity to the city's miniscule Main Street, which is so small that some Yorba Linda residents do not know it exists. The problem, however, is that some of the most active areas of tiny downtown Yorba Linda are found on the opposite side of Imperial Highway, a six-lane thoroughfare with a median. Here are Mimi's Restaurant, one of the strongest local draws, the Station Shopping Center (named for the former Red Line station, still standing) and Nixon Park, the largest downtown green space and the place most capable of holding crowds. Located a few blocks southwest (not visible on our maps), the Nixon Library is by far the biggest regional draw, and the city eventually wants to strengthen the pedestrian link between the Nixon Library and Main Street. Fortunately, downtown Yorba Linda is "almost all right," to borrow a phrase from Robert Venturi. The area is rich in historic buildings to lend scale and texture to new development. And with some help, the streets can approximate a more-or-less regular grid. Master plan architect David Denton, a contractor of the city's primary downtown consultant, Downtown/Main Street Visions, has centered his efforts on Main Street. The modest, two-block-long downtown stretch comprises mostly two-story buildings, with some vacant lots intervening. Denton proposes to fill in what he calls the "missing teeth" on Main Street with compatible new construction. To create a uniform retail strip, Denton would like to reconfigure some buildings that are currently in use as professional offices, with retail at street level and offices above. To the south, he proposes extending the two-story scale of Main Street into "large floor plate" retail space capable of accommodating big retailers. The scale of these new buildings would echo the scale of the packing plants that formerly occupied the site, according to Denton. Fronting on Imperial Highway, the new retail buildings would be a kind of billboard for Main Street, which is otherwise invisible from the thoroughfare. Historic buildings line the streets both east and west of Main Street: To the west is the existing public library, where Denton would like to remove the surface parking and replace it with structured parking. An existing historic house on the site would be remade as a historic museum or a children's museum. (I vote for the latter, which would be more of a draw and has the potential of repeat business.) To the east is School Street, with a row of historic houses, and further still is Valencia Avenue, where the master plan envisions multifamily buildings. The bridge would be located between Main Street and the new retail buildings. Although Denton said he is normally a skeptic about pedestrian bridges — and he actually advised another city against just such a solution — he said Yorba Linda's special conditions will make the bridge acceptable to pedestrians. To start with, the Main Street side of the bridge starts on the second level of the new retail building. In other words, people on foot are not obliged to climb a set of stairs to an isolated bridge. On the park side, the bridge ends in a stepped ramp. Popular in Europe but rare here, the stepped ramp is a wonderful urban-design device: With an individual step every few feet, the ramp creates a steeper slope than a conventional ramp in an unobtrusive way. And rather than being only a corridor, the master plan sees the ramp as a destination in itself, with a sculpture garden on permanent display. The bridge is essential for the choreography for downtown festivals: Starting on Main Street, the processional would move south through the new retail buildings and cross the bridge, which is wide enough to line with booths. The parade ends up at Nixon Park, which would be fitted with a bandstand and fountains. If those conditions do not entirely take away the discomfort of the pedestrian bridge, they may provide the incentive to make the trip in the first place. And if Yorba Linda's gambit succeeds, the city will have accomplished the rare feat of extending its downtown across a busy road packed with through traffic. Noise aside, it may turn out that what lies on either side of a bridge is more important than what happens in between.