The Doctor takes Martha to New Earth, thirty years after his last visit. Something has gone very wrong: there's a massive traffic snarl on the Motorway, a conduit beneath New New York, and the congestion has trapped people in their vehicles for years. When Martha is kidnapped by a desperate couple who need her to gain access to the Motorway's fast lane, the Doctor sets off in pursuit with the help of a Catkind named Brannigan. But rumours abound that there are creatures in the depths of the Motorway who prey upon the fast lane. And, elsewhere on New Earth, the Face of Boe is waiting with a final message for the Doctor...

When he became the executive producer of Doctor Who, Russell T Davies knew that a problem he would have to confront was the difficulty in establishing an overall sense of continuity for the audience, given that the TARDIS tended to land somewhere new in every story. A partial remedy was to introduce recurring characters for those episodes set on present-day Earth. As a further measure, Davies sought opportunities to revisit other settings from time to time. Having already visited the Year Five Billion in both 2005's The End Of The World and 2006's New Earth, he decided that a return trip to the far-future home of humanity should occur during Season Twenty-Nine.

A key decision Davies had made when planning Season Twenty-Seven's revival of Doctor Who was to establish the premise that the Doctor was now the last of the Time Lords. However, he had always intended that this would eventually be revealed as a deception, should the programme continue for a suitable period of time. In fact, Davies had strongly hinted at this development in a piece he wrote for Panini Publishing's Doctor Who Annual 2006, released soon after his first season ended in 2005. Originally, the Face of Boe was to impart this information with its dying words at the conclusion of New Earth. However, when the BBC confirmed that Doctor Who would continue into 2007, Davies decided to keep the Face of Boe alive and unravel the mystery more gradually. New Earth would now merely foreshadow the deathbed revelation, setting the stage for 2007's Year Five Billion episode.

Russell T Davies was entertained by the notion of resurrecting a nearly-forgotten 40-year-old monster

From the outset, Davies wanted to avoid writing a direct sequel to New Earth, and so he shifted the setting thirty years in the future -- to AD 5,000,000,053 -- and into the city of New New York, which had previously been glimpsed only from a distance. As initially conceived, the action would have been split between the wealthy inhabitants of the Overcity, the downtrodden denizens of the Undercity trapped on the Motorway, and the gargantuan creatures living in New Earth's oceans. When imagining an appropriate aquatic monster, Davies had considered both a reptile akin to the nuclear-powered Godzilla (the star of a series of Japanese kaiju movies which had begun in 1954) and a giant octopus. When his thoughts turned to enormous crabs instead, he realised that these could represent the future evolution of the Macra, aliens who had featured in the 1967 serial The Macra Terror. Davies was entertained by the notion of resurrecting a nearly-forgotten forty-year-old monster, especially since his story would not rely on any prior knowledge of their first appearance.

Meanwhile, the development of the episode -- called Gridlock -- had begun to focus more and more on events in the Motorway. This prompted Davies to eliminate the dwellers of the Overcity as living characters altogether. Furthermore, the bottom of the Motorway would no longer give onto the seas of New Earth, but would instead be a dark region filled with noxious exhaust fumes. As a happy coincidence, however, this further supported the use of the Macra, since The Macra Terror had already established their need to live in a gas-rich environment. One story strand excised at this point involved baby Macra, which were feeding upon the drivers who used the Motorway's fast lane.

Several elements of Gridlock were directly inspired by the comic book 2000 AD, an anthology which had been running since 1977. From the Judge Dredd strip, Davies seized upon the overcrowded Mega-City One for the look of New New York; the dystopian 1982 film Blade Runner would also become an influence. Dredd's informant Max Normal suggested the appearance of the bowler-hatted Businessman, while reporter Swifty Frisko from The Ballad Of Halo Jones gave Davies the idea for Sally Calypso. On the other hand, Brannigan's aviator-influenced look was taken directly from Ratz, a character created for BBC children's programming in 1994. The element of the mood drugs came from BBC Books' 2005 Ninth Doctor novel Only Human; it was written by Gareth Roberts, who was developing The Shakespeare Code for Season Twenty-Nine.

One of Russell Davies' goals in writing Gridlock was to provide Novice Hame with a form of redemption

Directing both Gridlock and The Lazarus Experiment, which together formed the year's third production block, was Richard Clark. Clark was relatively new to television drama, but had impressed Doctor Who executive producer Julie Gardner with his recent work on two episodes of Life On Mars. Amongst his cast was Bridget Turner, playing Alice Cassini; she was married to Frank Cox, who had directed episodes of Inside The Spaceship and The Sensorites in 1964, for Doctor Who's very first season. Meanwhile, Anna Hope returned as Novice Hame, the Catkind character she had originated in New Earth. Davies had been very pleased with Hope's performance in that story, and one of his goals in writing Gridlock was to provide Hame with a form of redemption.

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. Millennium FX had upgraded the Face of Boe's mouth animatronics for New Earth, only for the character to speak telepathically throughout the adventure. As a result, it was agreed that his final secret in Gridlock would be communicated orally, so that the enhancements would not go to waste.

With the exception of an off day on September 24th, the next week was spent in Doctor Who's regular studio facilities at Upper Boat. Clark's main concern was the various car interiors, which all used redressed versions of the same set. It began as Brannigan and Valerie's car from September 20th to 22nd, and became the Businessman's car at the end of the third day. While the set was being altered, Clark also recorded the scene of Brannigan collecting the Doctor from the Motorway platform. The 23rd and 25th focussed on Milo and Cheen's car. September 26th saw the set serve, in rapid succession, as the cars driven by the Cassinis, the Red Man, and Whitey; material in the TARDIS was also recorded on this day. Finally, the 27th saw Clark complete the remaining car sequences -- more shots in the Businessman's vehicle, alongside those belonging to the teenaged girls and the naked couple -- in addition to the Doctor's descent from car to car in the Motorway, and various inserts.

The Undercity alleyway was actually The Maltings in Cardiff Bay; filming there took place on September 28th and 29th. Part of the latter day was also spent on the Doctor's pursuit of Milo, Cheen and Martha through the warehouse, which was recorded at the Ely Paper Mill in Cardiff. Work on Gridlock then wrapped up with three further days at Upper Boat Studios. On October 2nd, action was taped in both Ma and Pa's car and Javit's car, together with several inserts and effects shots. Ma and Pa were given costumes and make-up which intentionally recalled American Gothic, a 1930 painting by Grant Wood, albeit informed by the popular misconception that the image depicted a married couple, as opposed to a father and his daughter. A remount of the scene in the naked couple's car was filmed on October 18th alongside other pick-up shots for Gridlock. Finally, inserts of Sally Calypso in the viewscreen were completed on November 7th.

During post-production, the Face of Boe's dialogue was amended to have him refer to the Doctor as an “old friend”. Davies had now decided that the season finale, Last Of The Time Lords, would strongly imply that the Face of Boe was actually a far-future manifestation of the Doctor's immortal former companion, Captain Jack Harkness, and he wanted to seed this notion into Gridlock as well. The episode then aired on April 14th, when BBC One's evening schedule was modified to accommodate the FA Cup semi-final between Manchester United and Watford on Match Of The Day Live. The football match would lead into Gridlock, which was scheduled at 7.40pm, forty minutes later than usual.

• Doctor Who Magazine Special Edition #17, 22nd November 2007, “Episode 3: Gridlock” by Andrew Pixley, Panini Publishing Ltd.
• Doctor Who: The Complete History #55, 2015, “Story 181: Gridlock”, edited by John Ainsworth, Hachette Partworks Ltd.