Investors list valuations as their primary concern for the infrastructure market, according to Preqin.
A survey by the data analysis company found that the proportion of investors worried about current valuations of investments rose from 13% in the first half of last year to 56% in the second half of 2015.
The research also found that the average deal size has risen 56% since 2013.
Transactions completed in 2013 had an average size of $401m (€363m), compared to $626m for deals done so far in 2015. The number of deals taking place each year has fallen notably in three years, from 1,056 in 2013 to just 325 so far in 2015.
“Growth has not been completely linear across the industry,” said Andrew Moylan, Preqin head of real assets.
Transactions in North America and Europe have seen the largest increases in average deal value, Moylan said.
“Appetite for the favourable characteristics of brownfield sites among investors has driven prices for these assets up at a faster rate than infrastructure at both the greenfield and secondary stages.
”What worries investors is that capital committed now may not deliver the strong, stable returns to which they have become accustomed.”
Moylan said: “Only time will tell whether large asset prices will have an effect on the overall performance of unlisted infrastructure funds currently investing capital”.
The study found that transport, telecoms and energy markets all saw notable rises in average deal size over the past year, with the average transport deal size for this year reaching $889m, a 32% increase compared to 2014.