"When the weather's really bad like it was [yesterday], the German car had to slow down to about 50km/h," he said.
"Even though they've got support vehicles all around them, basically the traffic stuffed them and the police actually stopped both cars because they weren't going fast enough."
Today the German team will enjoy a cultural day in Hamilton, which will be hosted by Waikato University and will end in a "nice Kiwi barbecue", Dr Duke said.
From there, both cars will slowly make their way to Bluff.
They expect to be travelling 150km a day at about 70km/h before the battery-powered car needs to be charged for between two and four hours.
Dr Duke said they were not in any hurry to get to Bluff but hoped to get there by early December.
The SolarWorld GT needs sunny weather to be powered, so cloudy days inevitably slow it down.
From Bluff, the SolarWorld GT will head back up to Christchurch and from there it will be flown to the United States in order to continue its journey around the world.
Once it has driven from the West Coast of North America to the East Coast, it will go on to Africa, Europe and Asia.
In all, it will travel more than 34,000km across five continents and two equator crossings, which will make it the first "World Wide Solar Ride".
The University of Applied Sciences Bochum said the SolarWorld GT's journey was proof of the "durability and capability of solar-powered vehicles".
