<P> A 1987 study said that crash involvement significantly increases when trucks drive much slower than passenger vehicles, suggesting that the difference in speed between passenger vehicles and slower trucks could cause crashes that otherwise may not happen . In a review of available research, the Transportation Research Board said "(no) conclusive evidence could be found to support or reject the use of differential speed limits for passenger cars and heavy trucks" and "a strong case cannot be made on empirical grounds in support of or in opposition to differential speed limits". Another study said that two thirds (67%) of truck / passenger car crashes are the fault of the passenger vehicle . </P> <Table> <Tr> <Td> </Td> <Td> This section does not cite any sources . Please help improve this section by adding citations to reliable sources . Unsourced material may be challenged and removed . (November 2014) (Learn how and when to remove this template message) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> This section does not cite any sources . Please help improve this section by adding citations to reliable sources . Unsourced material may be challenged and removed . (November 2014) (Learn how and when to remove this template message) </Td> </Tr> <P> The basic speed rule requires drivers adjust speeds to the conditions . This is usually relied upon to regulate proper night speed reductions, if required . Numeric night speed limits, which generally begin 30 minutes after sunset and end 30 minutes before sunrise, are occasionally used where, in theory, safety problems require a speed lower than what is self - selected by drivers . </P>

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