On Monday morning, the first cars crossed the westbound span of the new Highway 520 bridge, a milestone that took decades for politicians and engineers to conceive and complete. Here’s a brief look at how we got here.

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The first cars zoomed westbound across a brand-new 520 bridge Monday morning, marking the beginning of the end of a $4.5 billion project that began in fits and starts decades ago.

The process of building the world’s longest floating bridge has been like those of many other Seattle-area megaprojects: The design was brokered amid contentious politics, its execution was slowed by problems during construction, and taxpayers ended up paying a higher-than-expected price.

Here’s the (recent) history of how we got here:

For years, politicians wrangled over how to fund a replacement for the aging bridge over the northern part of Lake Washington.

• In 2007, voters rejected a large transportation package that would have included some funding for the bridge, leaving local officials, the governor and state lawmakers at loggerheads over the size of the span, whether it should be equipped for light rail and how to fund its construction.

• In 2011, a panel of local politicians (except for former Seattle Mayor Mike McGinn), approved a new, six-lane bridge. The plan got federal approval in August, and a contractor signed on in the fall.

• In December 2011, tolling began on the old bridge, which was constructed in 1963. The tolling became a subject of much public consternation as the state struggled with its billing system.

• In May 2012, the bridge contractor ran afoul of state law when a KOMO news investigation showed workers drinking at the bridge’s job site. Five managers were suspended.

• In the fall of 2012, the state discovered cracks in the bridge’s pontoons, which were caused by mistakes state engineers made during the design process and cost millions to fix. The following spring, the state fired its top bridge engineer, who allegedly took shortcuts in reviewing the pontoons’ design.

• By January 2014, the project was so over budget that then-Transportation Secretary Lynn Peterson had to ask lawmakers for more money.

• In March 2015, a 34-year-old bridge worker died in a fallon the bridge, one of several worker-safety incidents that drew attention to the project as the bridge was constructed. The following week, a load of steel pipe, which was being lifted by a crane, swung out of control and hit a bus traveling on the old 520 bridge, as well as a sign that fell onto the bus. The accident injured the bus driver and seven passengers.

• In August 2015, a crane crew lifted the final pieces of the bridge’s road deck onto its floating base.

• This month, the state opened the bridge to crowds on foot. A dearth of shuttle buses for the estimated 25,000-30,000 visitors caused the bridge’s first “bottleneck,” as a Washington State Department of Transportation spokeswoman called it. People spent hours waiting to get off the bridge. A day later, cyclists got to travel across the main bridge deck as part of a Cascade Bicycle Club ride.

More is still to come.

On April 25, the bridge will open to eastbound travelers.

Cyclists won’t be able to cross the bridge in bike lanes until next year, when a connection to Montlake is completed.

The state plans to begin construction by 2018 on the stretch of roadway between the new 520 bridge and Interstate 5, including pedestrian and bike paths over Interstate 5, highway lids and a new Portage Bay Bridge.