The Doctor takes Martha to New Earth, thirty years after his last visit. Within minutes, Martha is kidnapped and finds herself a captive on the Motorway, a seemingly endlessly congested traffic conduit on which thousands of people have become trapped -- in some cases for decades. And rumours abound that there are creatures in the depths of the Motorway, living... and feeding.

Executive producer Russell T Davies was aware that a potential problem with Doctor Who was the difficulty in establishing a sense of continuity with the audience, given that the TARDIS tended to land somewhere new in every story. He had addressed this, to an extent, by devising recurring characters for those episode set on present-day Earth. As a further measure, Davies wanted to have some other settings recur from time to time, and so for Doctor Who's 2007 season, he decided that a return trip to the eponymous planet of the previous year's New Earth was in order.

Davies had originally planned to set a key story arc in motion in New Earth by having the Face of Boe reveal with its dying words that the Doctor was not the last of the Time Lords after all. When Davies was informed that Doctor Who would continue into a third season early in the planning stages for the 2006 episodes, he elected to defer these developments, using the Face of Boe's appearance in New Earth to foreshadow it instead. The crucial event would now take place in the 2007 New Earth episode.

From the outset, Davies wanted to avoid writing a direct sequel to New Earth, and so shifted the setting thirty years in the future (to 5,000,000,053 AD) and into the city of New New York, which had only been glimpsed from a distance before. As initially conceived, the action would have been split between the wealthy inhabitants of the Overcity, the downtrodden denizens of the Undercity riding the Motorway, and the gargantuan creatures living in New Earth's oceans. In devising an appropriate aquatic monster, Davies had considered both a Godzilla-like reptile and a giant octopus, before recalling the enormous crab-like Macra which had featured in the 1967 serial The Macra Terror. Davies was entertained by the notion of resurrecting a forgotten forty year-old monster, especially since his story already called for exactly such a beast.

Meanwhile, the development of the episode -- called Gridlock -- had begun to focus more and more on events in the Motorway. This led to Davies eliminating the dwellers of the Overcity as living characters altogether, and moving the Macra from New Earth's seas to the depths of the Motorway itself. As a happy coincidence, The Macra Terror had already established the Macra's need to live in a gas-rich environment, which dovetailed nicely with the exhaust-choked Motorway. One story strand excised at this point involved baby Macra, which were feeding upon the drivers of the Motorway's lowest level.

Several elements of Gridlock were directly inspired by the comic book 2000AD. From the Judge Dredd strip, Davies seized upon the overcrowded Mega-City One for the look of New New York, and upon informant Max Normal for the appearance of the bowler-hatted Businessman. Reporter Swifty Frisko from The Ballad Of Halo Jones gave Davies the idea for Sally Calypso. In addition, Brannigan's appearance was taken directly from Ratz, a character created for BBC children's programming in 1994.

Directing both Gridlock and The Lazarus Experiment, which formed the year's third production block, was Richard Clark. Clark had recently handled episodes of Life On Mars. Amongst his cast was Bridget Turner, playing Alice; Turner was married to Frank Cox, who had directed episodes of Inside The Spaceship and The Sensorites for the original Doctor Who series' very first season.

Production on Gridlock began with two days at the Temple of Peace in Cardiff, on September 18th and 19th, 2006. This was the site of the Face of Boe's retreat; the clips involving Sally Calypso were also taped there. With the exception of an off day on September 24th, the next week was spent in the studio at Upper Boat, covering sequences in virtually all the cars and the TARDIS interior, more material in the Face of Boe's lair, and greenscreen work. The Undercity alleyway was actually the Maltings in Cardiff Bay; filming there started on the 28th and continued to the 29th, when the Doctor's pursuit of Milo, Cheem and Martha through the warehouse was also recorded at the Ely Papermill in Cardiff. Work on Gridlock then wrapped up with further days at Upper Boat, with sequences inside the cars completed on October 2nd, and inserts and remounts undertaken on October 18th and November 7th.

• Outpost Gallifrey: New Doctor Who Series News Page, edited by Shaun Lyon.
• Doctor Who Magazine Special Edition #17, 22nd November 2007, “Episode 3: Gridlock” by Andrew Pixley, Panini Publishing Ltd.