UK drivers are being urged to look for the cheapest petrol and “to fill up as usual” as travellers prepare to make 21.7m journeys on what is expected to be the busiest Easter on the roads in four years.

The average price of a litre of unleaded petrol rose by 20p in March, from 132.83p on the 1st to 152.83p on the 31st, raising concerns about the cost of filling up for Easter journeys. The higher fuel prices have been triggered by rising oil prices as a result of the US-Israel war against Iran.

It was the fastest monthly price rise for petrol on record, according to data released by the motoring services company RAC, surpassing the previous record increase of 16.6p in June 2022, after Russia’s invasion of Ukraine.

But while the RAC said the jump was “unprecedented”, its head of policy, Simon Williams, urged travellers to continue their journeys while looking for the best deals. “Ahead of the Easter getaway, which is expected to be the busiest on the roads since 2022 … we urge drivers to fill up as usual and to … find the cheapest forecourts near them.”

National Highways are planning to temporarily suspend 1,500 miles (2,400km) of roadworks from motorways and major A-roads in England between Thursday and Easter Monday, as part of efforts to ease journeys, as the AA predicted 1m additional trips would be made compared with last year.

That will result in 21.7m total journeys, making this Easter bank holiday weekend the busiest on the roads since 2022, when travellers made their first full getaway after the end of Covid lockdowns. Traffic is predicted to peak on Thursday, when many schools break up for the holiday.

Some analysts suggested travellers may choose to cut back on their spending over Easter, including on fuel, amid the surge in petrol and diesel prices. Susannah Streeter, the chief investment strategist at the Wealth Club investment service, said: “There may be shorter trips planned ahead, and fewer chocolate treats bought along the way.”

The AA said that just over half of travellers expected to travel less than 50 miles this weekend, with 5% planning trips of 50-100 miles. About 1% expected to travel 100-200 miles, with fewer than 1% planning to clock up more than 200 miles.

There were likely to be more cars on the motorways due to train disruptions, with engineering works taking out west coast mainline services between London Euston and Milton Keynes from Good Friday to Wednesday 8 April.

There will be no rail service between Preston and Lancaster on 4-5 April, while in Hampshire no trains will run between Winchester and Southampton, and in Kent there will be no services between Herne Bay and Ramsgate. Services between London Waterloo and Clapham Junction will also be reduced.

Many will be choosing to go abroad, with the travel trade organisation Abta estimating that 2 million people from the UK will travel overseas this weekend. EasyJet said it was preparing for its busiest Easter getaway yet, reporting it would run 16,000 flights from UK airports during the two-week school break.

Those going to the European Union are being warned to expect two-hour delays as countries deploy a new border system, known as the EU’s Entry Exit System. It requires those from third-party countries, such as the UK, to have their photograph and fingerprints taken in order to enter the Schengen area.

It is a situation experienced by many motorists: one driver overtakes another only to find the slower car is right behind them when they reach a red light. Now a researcher has used mathematics to reveal why the situation feels inevitable.

Dr Conor Boland from Dublin City University has called his work “The Voorhees law of traffic”.

The name is a nod to the character Jason Voorhees from the American horror film franchise Friday the 13th.

“I always thought of him because he seems to just walk everywhere … His victims are running away, they’re sprinting, but he just catches them,” Boland said.

However, in the case of cars it is traffic lights rather than stones or tree branches that prevent the “escape” of the faster individual.

Writing in the journal Royal Society Open Science, Boland explains that as two cars travelling at different speeds encounter a traffic light, the spacing between them can either increase, stay the same, partly decrease or be completely lost, depending on the colour of the traffic light, its duration, the time advantage of the faster car and the overall time it takes for the traffic lights to complete a cycle.

Assuming that the traffic light changes colour based on a set time cycle rather than a sensor, and that the cars are travelling on a single-lane road, the results reveal that, taking into account the probabilities of each of the four scenarios, on average the possible gains and losses in spacing between the cars balance exactly.

That means, on average, the lead of one car over the other remains the same after the light as before.

The results suggest the idea the slower car will inevitably catch up at the lights is something of an illusion.

“Recurrent encounters are known to be disproportionately salient in human perception, particularly when they follow an attempted separation or avoidance,” Boland writes.

Put simply, it is probably just more memorable for a driver when the car they overtook catches up with them again.

However, when drivers encounter a succession of independent traffic lights – as might occur in towns and cities – the situation is different.

As Boland explains, here the eventual catchup of the slower car at at least one of the lights becomes statistically near-certain. That’s because the probability that the slower car never catches up relies on multiplying the probability of no catchup for each traffic light – meaning the more lights, the less overall chance no catchup will occur.

Boland said the results have implications for road safety, suggesting speeding past others does not necessarily give an advantage.

Kit Yates, professor of mathematical biology and public engagement at the University of Bath who was not involved in the work, welcomed the study.

“It’s something I, as a slower driver, often think about. Was it really worth it for that car that sped to overtake me? When I catch them up at the lights I smugly think, ‘No, it wasn’t.’ So it’s good that someone has sat down and modelled how and when this happens,” he said.

However, Yates added that in his experience, catching up to a faster car does not seem to happen constantly, or to be surprising when it does occur.

He also noted that the study made a number of assumptions, among them that cars travel at constant speed between lights with no acceleration when a light changes to green or deceleration when it changes to red.

“But as the old adage goes, ‘all models are wrong, but some are useful’ and I think this one is definitely useful to explain why slower cars can often catch up with quicker ones,” Yates said.

The UK aviation regulator has partially rejected plans by Heathrow to significantly raise its landing fees to fund a multibillion-pound upgrade, arguing the airport can still invest without steep rises to ticket prices.

The Civil Aviation Authority said the average charge for each passenger should rise from £28.40 to £28.80 between 2027 and 2031.

Last year, Heathrow proposed a 17% increase to £33.26, which resulted in criticism from airlines who said it would lead to higher ticket prices for passengers.

The CAA said its average passenger charge would instead rise by 1%. That increase is £5.40, or 16%, lower than the changes proposed by Heathrow but significantly higher – £5.80 or 25% – than the changes wanted by the airlines.

Selina Chadha, group director of consumer markets at the CAA, said: “Our primary duty is to protect consumers and at the heart of today’s proposals is doing the right thing for passengers using Heathrow airport, while supporting sustainable growth, investment, and efficiency.

“Our proposals for the airport charges levied by Heathrow on airlines strike the right balance between keeping passenger prices fair, while enabling the airport to make the investment needed to improve services for the future.”

In its initial plans published on Tuesday, the CAA proposed Heathrow spend between £5.4bn and £6.1bn on projects including upgrading the airport’s electrical system. Last year, Heathrow was forced to close after a fire at a nearby electricity substation caused a power cut, resulting in the cancellation of more than 1,300 flights.

Europe’s busiest airport had been seeking approval to spend up to £10bn to handle an extra 10 million passengers a year by 2031, with upgrades including a plan to modernise Terminal 5.

Thomas Woldbye, the chief executive of Heathrow airport, said: “We will now review the CAA’s initial proposal in detail to fully understand the implications for delivering the innovation, progress and improvements customers expect.

“On the face of it, the CAA’s proposal may force choices that create trade-offs for service and delay delivery.”

The CAA will publish its final proposals in November, which do not include its plans for a third runway, with a final decision expected in April 2027.

In 2023, Heathrow was forced to cut passenger charges by almost a fifth after losing an appeal to the UK competition watchdog against the CAA.