Roots: Blade Runner, The Fifth Element, and Star Wars. The communications system between the cars is inspired by social networking sites like MySpace and Facebook. The moods were inspired by the Doctor Who novel Only Human. The hymns that are sung are 'The Old Rugged Cross' and 'Abide with me'. The three passengers to enter the fast lane rule is inspired by "car pool" lanes or "multi occupancy vehicle" lanes, which are designed to cut congestion and encourage car sharing.
Goofs: In New Earth, the Doctor said that New Earth is in the galaxy M87, which is a real galaxy in the Virgo cluster. It is half a million light years across and 60 million light years away. Yet here the Doctor says that New Earth is 50,000 light years from Earth. In 5 billion years the Milky Way and M87 ought to be millions of light years further apart, not closer together. [Perhaps the people of the year 5 billion have the ability to move solar systems and galaxies around.]
If the there is self-replicating fuel, then how can the three passengers to go in the fast lane rule be meant to save fuel? And how does the self-replicating fuel work anyway? Surely the second law of Thermodynamics would mean that you need some external input to keep your fuel supply going for that long. Especially if all the cars are generating exhaust fumes (and hence losing some of their fuel as part of the process of using it). The same goes for recycling all the waste products into new food. Some of the food will get used up as energy, even if you lie comatose when not eating it.
If it takes 6 years to go 10 miles, why don't people just walk? [Everything is sealed off, so the motorway is the only way to get anywhere.]
What did the carjackers mean when they talked about being "too poor" to have a third passenger?
When the car turns off all its systems, including the engines, how does it stay floating? [Perhaps they've invented a material that naturally repels gravity. Although that begs the question of how they get one of these cars to land.]
If the undercity was sealed off, then why is there both sunlight and rain in the section that the Doctor and Martha arrive in? [It's artificial].
And how can a virus possibly infect an entire planet in just seven minutes? [Either they have a special virus-spreading teleportation system, or the planet is very sparsely populated, and the virus was teleported right into the heart of its population centres.]
Continuity: Streets of New New York include Junction 509 and New Fifth Avenue and areas include Fire Island, Battery Park, and New Brooklyn. Sally Calypso is a hologram who does traffic reports. Cars carrying three adults get special access to the motorway. Legend says that, back in the old days, a woman stood in the exhaust fumes for 20 minutes at junction 47 and her head swelled to 50 feet. The motorway is entirely self-enclosed. Traders in the New New York undercity sell moods, including happy, anger, mellow, forget, sleep, and honesty.
Technology in the cars includes self-replicating fuel, muscle stimulants for exercise, chemical toilets, and recycling all waste products for food. You can contact people in other cars, but only if they are on your friends list. The daily contemplation includes hymns such as The Old Rugged Cross and Abide with Me.
The Doctor describes Gallifrey as having burnt orange sky, with the Citadel enclosed in a mighty glass dome, shining in the twin suns. The mountains beyond go on forever with slopes of deep red grass capped with snow. He is evasive about the fact that Gallifrey has been destroyed. He says that the second sun would rise in the south and the mountains would shine. The leaves on the trees were silver, and when they caught the light every morning, it looked like a forest on fire.
Martha doesn't know how to fire a gun.
Cat people and (new) humans can interbreed. At two months old, their children look like kittens.
Legend says that the Face of Boe has lived for billions of years. It says that he will speak his final secret to a traveller. He calls the Doctor "old friend". He says that he is the last of his kind, just as the Doctor is the last of his. His final secret, which he says before he dies, is "you are not alone".
Links: This story is a sequel of sorts to New Earth. The Face of Boe previously appeared in The End of the World, The Long Game, and New Earth. Martha mentions Rose. The Macra previously appeared in The Macra Terror. The Doctor mentions the Time War. The Doctor's line about "finding my own way, I usually do" may be a reference to a similar line uttered by Ian in Only Human. After New Earth, Novice Hame was made the Face of Boe's nurse as penance for her sins. The Doctor's description of Gallifrey is very similar to Susan's given in The Sensorites and the twin suns were first mentioned in Goth Opera.
Extras: This story has an episode of Doctor Who Confidential. The BBC website had an online commentary, which is no longer up. It also has a behind the scenes video podcast.
Location: New New York on New Earth, the year 5 billion and 53.
Future History: New Earth is 50,000 light years from Earth, New New York was ruled by a Senate. Novice Hame worships the goddess Santori. 24 years ago everybody died. A new mood called bliss killed everybody. A virus mutated inside the compound and became airborne. It killed the world in seven minutes flat, and then died itself. There was enough time to seal off the undercity. The only survivors were the Face of Boe and Novice Hame - who survived because the Face "shrouded her in his smoke". The face wired himself into the mainframe, giving his lifeforce to keep things running. The last act of the Senate was to declare the planet unsafe. The automatic quarantine lasted for 100 years.
The Macra used to be the scourge of this galaxy. They fed off gas - the filthier the better. They built up a small empire using humans as slaves. They have devolved over the billions of years since this time.
Unrecorded Adventures: Janis Joplin gave the Doctor his coat.
The Bottom Line: 'Everyone goes to the motorway in the end.' Gridlock is a well-drawn character piece. The Doctor and Martha's relationship is seriously fleshed out, even though they spend almost all the story apart from each other. The minor characters are all interesting in their own right, and the Macra certainly surprised me on first viewing. However, much of the story's success comes from its somewhat oddball status, as the plot is full of surprise twists that keep you guessing. On repeat viewings, it's nowhere near as exciting as it was first time round, and the plot feels a little pedestrian. By this point in the series, the constant mentions of Rose are getting a little tiresome, and the Face of Boe simply isn't interesting.