National Highways Authority of India is a Statutory Body established under the National Highways Authority of India Act, 1988. On 1st April 2019 NHAI issued non-convertible, redeemable, taxable bonds in the nature of Debentures with benefits under section 54 EC of Income Tax Act, 1961 for long term capital gains with a lock-in period on 5 years. The face value of each bond is Rs. 10,000. The investors can invest maximum Rs.50,00,000/- subject to other conditions. The interest rate on these capital gain bonds is 5.75% payable annually. Interest earned from these bonds is taxable. However, no TDS is deducted on interest.
The bonds issued by the NHAI are rated AAA by CRISIL and CARE which prominent rating agencies of India. AAA is the highest rating that may be assigned to an issue of bonds by a major rating agency. Debt issuers with the highest ratings have the strongest capacity to repay the debts.

These bonds are AAA rated and are backed by the government. Hence, the risk of interest and capital payment is protected. This rating indicates the highest safety with regard to the timely payment of interest and principal. The interest on these bonds is payable on the 1st bank working day of April 2020 in respect of bonds issued up to 1st Feb 2020.
So as per rating given by the Agencies these bonds are safe to invest.